Regarding the balance issues between WW, vamps, and those who are neither, it occurred to me that the main balance issue with regular mortals has resulted by the gradual shift in builds and skills owing to the Champion system. Originally, the Fighter's Guild skill line was a pretty good offensive tool against vamps/ww in pvp owing to the procs and extra damage they did to them. After the Champion system was developed, the skills were falling behind in utility and were updated for STAMINA builds to benefit from their investments in the Champion constellations. The skills are still much weaker than they once were and the gradual power creep hasn't help their utility all that much. As of right now, they're pretty much useless outside of pve and only in specific pve at that.
I understand that it's the FIGHTER'S guild, but even still the overall degradation of the skill line has led to quite a balance problem. On the other side of the table, WW and vamp are somewhat lacking in overall utility (vamp less so) and changing the skills every so often hasn't really improved the situation. Changing the skills here and their really only benefits a small amount anyway.

I also particularly hate that vamps are basically magicka builds and WW are stamina and would like to see them be more open to either. Or both.

So, the best long term balance solution, imho, is to set constellations in the Champion system similar to the ones in Skyrim. (I reallllllllly hate to blatantly pull from Skyrim, but the Champions system is basically ripped from Skyrim...) Passives are all on one constellation, the constellation changes when you become ww or vamp. Points are refunded if you change.
Basically the three constellations would be one for Meridia, one for Molag Bal, and one for Hircine.

The Meridia constellation should focus on damaging and daedra, undead, and werewolves. The unlocked points in the constellation could look something like the Fighter's Guild originally did. (I particularly miss the old Evil Hunter and crossbow.) The percentage fills should focus on dealing more damage and receiving less damage from undead/daedra/ww.

The Molag Bal constellation (or whatever it's called) should focus increasing health drains, debuffing enemies that have been fed on, etc. Unlocked points should have passives that focus on feeding and draining. Percentages should give higher health restores from feeding, resource restores from feeding/draining, reduced damage from fire, etc.

The Hircine constellation should focus on increasing bleed damage, longer time in WW, and buffs while WW. health restores from feeding, etc. Unlocked points should increase Bleed damage, increase resistances while in WW, etc. At least one of them should let WW's stack bleeds and inflict bleeds on constructs. The percentages should increase WW time, increase bleed damage, reduce damage from poison, etc.