I wholeheartedly look forward to what you put out there Pikka. I have loved playing with both UKRS (1+2) and NARS2 for many years.
As for suggestions based on the current game (mainly passenger operations) I am playing with UKRS2 I have found I lean to the following Units:
2-2-0 Planet - As I like to start in 1835
0-4-0 Coppernob - For additional pulling power and speed
0-6-0 Long Boiler -Because tractive power becomes my main issue.
4-4-2 Suburban - This becomes my 'standard' workhorse
4-4-0 Edwardian - Pulls my express trains as the faster locos are much more expensive.
2-6-4 Suburban - Replace the 4-4-2 Suburbans as they come to the end of their life.
English Electric Type 1 - Useful for branch lines and short freight ops.
Sulzer Type 2 - Fantastic loco that I throw at
Brush Type 4 - Powerful loco used for mix of passenger and freight.
EMD Class 66 - For later game replacements of the Brush Type 4
EMD Class 67 - Loco Hauled Passenger and fast freight.
Metro-Cammell DMU - First DMU is used to remove steam from branch lines.
Sprinter / Express Sprinter - Used as standard DMU to replace the 1st Gen and replace some of the Loco Hauled passenger work. I think the two classes could be merged though.
Turbostar - Modern DMU to replace the sprinter towards the end of the game.
BR HST - Fast Passenger DMU
Bombardier Voyager - Replace the HSTs
AEI AL1 / English Electric AL6 - The first 'proper' electric locos, I don't think both are really required though.
BREL Class 90 / GEC Class 91 - I use these as replacements for the above, again I think only one of these is really required.
BR AM10 - First Gen 25kv EMU
BREL Class 321 - Sometimes used to replace the above (where I want 100mph) but I often jump straight to the Electrostar, In some ways I would prefer to see this replaced with a Class 319 as the type would be more useful as a dual voltage unit.
Southern EMU - As my first 3rd Rail Type
BREL Renewal EMU - To replace the above.
Dual Voltage EMU:
Electrostar - Becomes my standard EMU for anything that isn't long distance express.
As the UK is going a little Bi-Mode crazy at the moment I would find it interesting to include:
Class 800/1/2 - Merged into a single unit to provide Bi-Mode express services.
Class 88 - As the last mile diesel engine would give something a little different to replace the Class 90/91
I didn't quite manage 10 with 23 that I use in the UKRS2 set with another 2 as wishes. I suspect the number is so high as I like playing big maps over long periods of time (~200 years) which is one of the reasons why I love UKRS/NARS. One other thing I would like to see is a GRF parameter to increase capacity on the suburban units for games played with cargodist to simulate standing room as I find it difficult to play some of the trains profitably.
Hopefully some of the above is helpful, I am really interested to know what other people consider to be the 'must' includes