Current problems:
1. The lack of filtering add-ons by category makes it hard to find add-ons with content that is interesting at the moment. The names themselves often say nothing. You have to check each add-on in the description, but even there is often missing the proper information.
2. Marks of the downloaded add-ons are barely visible so it's hard to spot new add-ons or their updates. You not clearly see what you already have and whats not.
3. The list of add-ons marked for download is missing. When sorting by download, marking an add-on to download every time will jump the list, which is quite a pain.
4. There is no way to download older versions - finding them outside of the game can be very difficult or impossible. Meanwhile, the information about replacing an add-on in the event of an update is false and misleading - it should inform about the update, about the new version of the add-on that a player already has, and not about its replacement. In addition, a player should know that he can select previous versions of add-ons in the game. The older ones can sometimes be better or more interesting than the newer ones, or simply without disadvantages.
5. Descriptions of add-ons and additional information are hard to read. In addition to changing the font colors, it would be useful to set an information layout that would be common and the same for all add-ons. An overview image of the add-ons would also be useful.
6. The location of the information on the total size of the files ready to download is confusing. I don't know what others think, but despite having known the game for several years, I still haven't got used to it - this location is unintuitive.
Solution proposals:
0. Putting all types of add-ons in one window
Such a solution would be more convenient and friendly, especially for new players who don't know yet how many different additions the game offers. For the same reason, it would be better to display all add-on types as tabs side by side rather than as a drop-down list.
Putting all add-ons in one window, but under separate tabs, would make it possible to remove duplicate and redundant add-on type designations here that could take more useful information.
1. Division of the entire list of add-ons into categories
Probably the most anticipated change. The arguments are the same as above, but here the form of tabs would be much more convenient than the drop-down list - here it would more often switch between categories looking for specific add-ons. In the case of a sufficiently wide window and short category names, it could be one line of tabs. It might or might not be a problem to assign categories to existing add-ons. This could happen gradually, not necessarily all at once. Add-ons not described would be placed in a separate group which will be removed over time.
2. Clearly marked add-ons already downloaded
Add-ons, the latest versions of which the player already has, would be marked with a gray, still legible but less distinctive color. Besides, the status icons could be clearer.
2b. Ability to hide add-ons already downloaded
This option would be especially useful for the
newGRF and the view of all categories. If a player had already downloaded most of the add-ons, this option would effectively shorten the list, making it easier to search among other add-ons.
3. A separate list for add-ons selected for download
It would just be useful. After searching the list for a long time, it is hard to remember all the add-ons marked for download. The dark box for this list would refer visually to the add-on settings window.
4. Ability to select and download an older version of the add-on
A player or add-on developer who for some reason would like or need an older version of an add-on would be able to choose it. It could be a selection from a drop-down list or from an additional window, with basic data in addition to the name.
5. Description layout more readable and uniform for all add-ons
The information part should always have the same layout, which would facilitate retrieval and readability. The division of information into categories would be an additional, significant facilitation. The background information would come from the system - every add-on have it and this information would always appear. The remaining categories would appear only if the add-on author puts their content in them. Their order would always be the same.
6. Information about the size of files in a more intuitive place
It seems most appropriate to include this information under the list of add-ons selected for download and on the same line under add-on description.
7. Marking of new additions and their updates
Where the current type designation of the additive is, an additive rating could be found. In the event of an update or new add-on, the rating for a month and when you first open the add-ons window would be hidden under the appropriate mark. In case of resignation from the possibility of rating add-ons, the marking could be a single letter in the place of the add-on download icon or before the add-on name.
8. Ability to group add-ons
Such a possibility could be the choice of the author of numerous add-ons. Groups of add-ons could also create or break up by supervisors as they deem appropriate. After clicking on the group bar, information about it and a description would appear. Double-clicking on the bar or single on the icon on the left will expand the list.
9. The most important information about the add-on visible at the top
In the case of
NewGRF add-ons, this could be information about the resolution as well as subcategories. For heightmaps and scenarios, this would be the map size. After stretching the window, further, less important information could be displayed, such as publication date, file size or number of downloads.
10. Ability to sort by many different ways
This form would allow sorting by categories that are not visible on top, such as dates. In the event that the extension of the window showed different data, this form of selection would be unnecessary.
11. Add-on preview
As they say, "One picture is worth more than a thousand words". If the data transfer were not a limitation, the possibility for the author to include a small graphical presentation of the add-on would be very helpful. Alternatively, to limit transfers each time an add-on is opened, pictures could be saved on the player's computer.
12. Guide
It could be useful for new players and those who returned to the game after a long break. You could include the most important information about add-ons here, as well as links to videos.
13. Possibility to evaluate add-ons
Very useful, but also probably the most time-consuming to implement and potentially controversial function.
- [+] Spoiler
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Possibility to evaluate - Why so
Information about an add-on is often missing, and the player doesn't know anything about it until they test it themselves. Meanwhile, there are very good, refined additions, which only lack the "Donate" option (FIRS, Squid, PKP and many, many others) as well as those that are missing a lot, which even spoil the game and where you would like to warn other players not to download it (e.g. CPU AI due to spamming with signs). That's why I think it would be nice to be able to approve of good projects or disapprove of those clearly underdeveloped. This would also give the authors information about what others think about their work.
Possibility to evaluate - Why not
There is some concern that these ratings could not be entirely fair for various reasons. The biggest fear is that players might judge the add-on on the spur of the moment. With heightmaps or scenarios, it doesn't take long to evaluate them. This takes time for most of the others. There could also be players, though this would rather be the minority, who would intentionaly judge the add-on negatively, for example just because the author didn't fulfill their wishes.
What can also be an argument for them is the fact that not everyone likes to be judged. While a positive evaluation can be encouraging, or at least neutral, with a negative one can be different. Some people will be encouraged to improve their work, but many such an assessment could be discouraging and simply unpleasant.
The introduction of a rating system could also have the effect of players focusing solely on highly rated add-ons.
There is also the question of players verification. I don't know anything about it, but I suppose that creating an effective system can be quite laborious.
Solutions
1. Verification
The basis could be an individual, unique identifier for each downloaded copy of the game. The next verification threshold could be the IP address, preferably matched with the number of the game copy - here the player who would like to evaluate the add-on could not use services or programs hiding its address. Another form of player identification could be based on the add-ons they have (ID + download dates).
2. The problem of being guided by single, extreme assessments
To avoid this, the ratings would be hidden for the first month after publication or update of add-on. Also, to avoid a situation where the rating of the add-on is only influenced by one or not much more extreme ratings, which may not be reliable, each add-on would initially be given e.g. 50 x 3 stars. Then, achieving a better or worse result would require obtaining a greater, significant number of similar ratings.
3. The problem of judging on the spur of the moment
The basic condition for evaluation would be to download the add-on and use it in the game. The evaluation could be made no earlier than 20 minutes after downloading the add-on. The player would always have the option to change his rating.
4. The problem of insufficient knowledge to reliably evaluate the add-on
Perhaps a good solution would be if only verified forum users with the appropriate seniority or rank could make the evaluation. There would not be many of these ratings, and perhaps not every add-on would be rated, but such ratings could be more reliable. Another solution could be to differentiate the impact of the rating depending on whether the player is registered on the forum or not, and if so, what rank they have.
5. Possibility to improve the rating of the add-on by the author
Each update would reset the add-on's rating - each version would have a separate rating, so no author or add-on would be doomed to eternal damnation. 
6. Alternate version 1
The grades could only be positive - each addition would start with a zero account and accumulate points over time. That number of points would mean another star.
7. Alternate version 2
The add-on could only be assessed by the player and only for himself. This could be the same form of a silver and gold star as for the NewGRF settings window. It would allow to mark additions important for the player. Then the ratings in both windows could be related to each other.