Xeviat
Dungeon Mistress, she/her
Kits the main one was weapon specialization and good saves. +1 to hit, +2 damage an extra atack ever second rounbd (starting from level 1) and by level 20 they had the equivalent of +15 to +18 on all saves (vs DC 20).
Weapon Specialization and Good Saves might be the thing I'd glom onto then. Extra crit range is fun, but it's random by it's nature and it stacks with other abilities in such was as to make those into optimal choices (and an optimal choice on a suboptimal choice kind of becomes the only choice to those informed, or at least the only strong choice). Bonus to hit could become problematic in 5E's flatter math, but 1 or 2 points across the levels might not be bad.
Giving Champions a saving throw bonus would be very interesting. It wouldn't compare with anything the Battle Master has, and could make them stand out.