I am sure a SE of England scenario will be popular. I think it would be best to release the scenario without any mods at all. Then people can play it with the standard configuration of Loco patched to date. Some people do not like playing with mods.
If you wish to also create a modded version with earth slopes and additional trains, then please create a second version of the scenario with a readme file specifying the exact mods you have used or included together with locations, if possible, as to where they can be found.
If you are using mods yourself, I believe it is necessary to "create" the scenario in a fresh installation of Loco without mods. This avoids some of the mods (like the earth slopes) creeping into your scenario which means people not having a particular mod cannot play your scenario as they receive "object not found" error messages when trying to load the scenario. If you read back through some of the scenario threads, you will see questions from people trying to load scenarios receiving the "object not found" error messages and that is frustrating for them
So please create a "clean" scenario first. Of course a list of suggested mods to play the scenario with is also welcomed.
I agree with dave61, your objective for the scenario could be either to move so many passengers - in x years if you wish, create a company of a certain value, or maybe a combination of these.
The only other point I would make on the scenario is don't make your towns too big to start. They will grow very quickly in-game with a good player