There is only so much that can be done with the troop tree if you want to keep the mod savegame compatible (which means vanilla troop tree structure and vanilla items). Vanilla design is 5 tiers of normal troops and tier 2-6 for nobles. To make units diverse under such conditions you must start at cloth armors and end at splint over mail very heavy armors. There is not much else that can be done with it other than is already done in RBM as we have our own overhaul that is balanced for combat module included in the combat module.There are some mods out there that do overhaul the troop tress quite a bit. Some even have patches for RBM compatibility now.
However, I think that once the game matures, it would be interesting to see your take (and the rest of the RBM team's take) on the Bannerlord troop trees.
However if I was to play with more radical overhaul I would pick DRM, since it is has most historically accurate gear and troops quality (lot of unit branches go only up to tier 3, which fits well with levies). RBM is more like hundred years war 10th-12th century edition when it comes to amount of good quality gear on troops.