Little things like vocal fry, end breaths, growl samples, extra falsetto voices, cross compatible aliasing, extra voice acted wavs, extra samples, that sort of stuff.
I definitely like banks that have multi-append/expression (whatever you wanna call it) I've listened to a couple like that and they sound really amazing. So cross-compatible banks definitely sound good to me.
Having different type of ending breathes, breathes, for VCV or CVVC "long tone vowel" recordings, glottal stops and fancy VC extras are fun to use - if voice bank has those sounds. :v
It's surprisingly common that people skips recording ending breathes(?!?) Breathes also misses often (which is kinda ok because I need them rarery). But ending breathes; those I like to use