@alkexr

Sorry for missing that you have an exception for plains! This changes things a bit but I think my thoughts still hold:

Stacking bonuses: stacking different bonuses is unaffected by the simplification I am proposing. It is still posible in exactly the same way, it is just easier because there are fewer categories.

Cleaning up the table: let us look at a 4/5 "poor" unit. If you change it to a 4/6 “poor” unit, to keep everything the same, you would need to change the river modifier to -1 and the plains modifier to -1 as well but all the other (defence) modifiers would be zero. If you do this for all units you get much cleaner stats. “Poor” units would have only one modifier (-1) for two terrain types and the rest would be zero. You are correct that it would not improve the situation for rivers and plains would be worse of (1/-1 instead of 1/0) but it would be much better for ever other terrain type.

Getting Rid of unit types part one: you are correct in that you cannot simply get rid of “excellent” and “good” unit classes if you make a special case for plains. But you can (and should) still get rid of both classes. You just need a tag that improves (or worsens) a unit in plains by 1. That would be a lot cleaner than introducing to separate classes of units which only differ in exactly this one thing from already existing classes. It would even be the same modifier. Formerly “excellent” and “good” units would receive a tag “maneuverable” (or something nicer sounding) which give them +1 to their defence on plains. Afterwards my original approach works again. Which was:

You do not need "excellent" units at all. Let us take your example: an excellent unit with 4/4 stats. You could give it a +1 baseline stat instead (making it a 4/5) and it would get a +1 bonus everywhere but on plains and rivers which makes it (almost) identical to a "poor" unit. You do this with all "excellent" units (and give them “maneuverable”) and you get rid of the whole category of "excellent" units and your matrix shortens by a whole row/line! With minimal changes to your logic. It would be a "poor" 4/5 unit (or a 4/6 “poor” unit if you implement the first change).

Getting Rid of unit types part two: you do the same for “good” units. You don’t even need a new tag, maneuverable works here as well.

This would be the resulting table changes:

(This includes merging territories that are identical.)

Sorry for coming back to this, I am a mathematician and these kind of things are just fun

@shorogyth Your solution to territory effects is certainly simpler and it would work fine if every territory had exactly one territory effect, but it's important that territories with multiple effects stacked (i.e. Caras Galadhon) should be easier to defend. Also I can't incorporate these bonuses into the baseline stats, because the plains bonus is attack only, while the rest are defense only. A 4/4 unit with excellent tactics acts as a 6/4 on plains and 4/6 on hills, I could technically make it a 5/5 but then the territory effects would need to be +1/-1 and -1/+1 and that's hardly a simplification. Maybe versatility is a better name than tactics though.

I admit the strategic role of charge is not very unique, it's essentially just "very good at attacking on plains, with a twist". But if I see my opponent buy charging units, I'm not worried for my cities, I'm worried that I'll be forced to retreat from open terrain and keep my army behind the lines because exposing them is a death sentence. I know what to be worried about, so my opponent has made a strategic choice.

Dragons can't evade, but the winged nazgul can, and yes, they are very scary.

As for complicated calculations, the battle calculator will do that for you if you need the exact numbers, but most of the time simple heuristics are enough. Charge is very good in plains, less so in hills or if there is a river etc., you don't need to memorize the every single number.

@mortetvie Anti-air attacks are still a thing. The balance obviously went out the window a long time ago, pretty much every unit will need to be rebalanced. There will also be quite a few new units, while some others will be retired. In general, it's too early to worry about balance at this stage.

@rogercooper
Cant find Ansalon/Analon on Github maps

Missing from your list, but on Github maps
Settlers: Fallen Empire
Shogun Advanced