I think this is a great idea. You explained it really well. It would help solve part of the issue with the high rank disadvantage. Right now how much you benefit depends completely on build with focus benefiting the most and builds like strength and caster benefiting the least. By forcing people to choose only half of the attributes to max, it makes it a lot more fair and also adds more depth to the system in terms of strategy.
New attributes like extra health or energy at the start of the battle, increased skill damage, decreased energy of skills, or faster Rage gain would be interesting. It would almost sort of be like bringing back passives.
I agree that making the rank cap 50 seems low, but I doubt they would ever make it so you only get a point every two ranks. It's been suggested a lot already, but I'd much rather just see a Rank 100 and Rank 200 Legendary shop be opened with some neat gear.
As for bosses, none require you to be any higher than Rank 50 that I know of. NPC Armor, Defense, Resistance, and Medical Mastery only require Rank 40. The rest are useless for any class except Tactical Mercenary, which benefits a tiny bit from NPC Crusher and Primary Damage for Frenzy. The only boss that might be too hard without them is Titan, but it would dfinitely still be possible. But, yeah, overall this shouldn't be a really big issue at all. Legendary bosses get indirectly buffed and nerfed all the time due to balance changes already.