First up, I really don't know much about how to mod, so I'm sorry if anything I say sounds ridiculously difficult to implement. I have played with a lot of mods on both M&B and Warband, though.
With the "Player Supporters Faction", is it at all possile to have a selection of perhaps 3-4 separate troop trees, allowing the player to select one to use when they begin their own faction. Perhaps one tree specializing in Infantry (with pikemen who have good defense Vs cavalry, light infantry for engaging archers and chasing down fleeing units & heavy units for breaching castle walls & breaking though other infantry , etc) , another in archery with a selection of bowmen for different situations (longbowmen interspersed with shortbowmen who fire faster but with a much shorter range, crossbows to punch through heavier armor, etc), another specialising in cavalry, another in something else ( I really dunno, I'm just spitballing).
I'd rather not have uber-powerful units which spoil the flow of the game, but I would like some more specialised units which allow for some more interesting tactics. If there are to be uber top-tier units, I'd prefer them to be very expensive to maintain, and very difficult to promote to. Otherwise, once you reach a certain economical level, you simply have an immortal/unbeatable army which I think takes all the fun out of the game.
Then I was thinking of a mix&match sort of thing where you can select what types of infantry, cavalry & ranged units you'd prefer to utilise, by ticking boxes or selecting them from a menu. Perhaps have a marshal npc to customise your personal troop-tree
e.g. If the player wants their top-tier infantry to be two-handed heavyarmor shocktroops, or onehanded+shield, etc.
Or top-tier archery units to have either much higher damage,range or ammo, or more defensive capabilities, or better skill at melee.
Sorry if this is fairly vague - I really had only a vague idea to begin with.
Oh, and apologies if this is the wrong forum to post all this in. I think I got carried away with it.