Increase the amount of sources the game draws from, including mordenkainen and volo, for character backgrounds
Restore reaction spells to abjurers, incl durability of magical shields, and ensure that monsters like the Beholder Spectator actually get their reactions/Legendary actions abilities (dragons get them too)
include multiattack for creatures/PCs that can actually use it (Lae'zel should be able to, as all Gith warriors as per MM can do two attacks), as should the Intellect devourer. Astarion kind of does already. Thugs, however, don't.