it has always annoyed me that i can give my pets heterochromia, but not my sims. we should have the same options for sims!
and i also really hate that we cant customise hair colours, like we can with makeup colours. especially the two-toned hair options, we really should be able to choose the primary and secondary colours (and tertiary, for the few that have three colours). similarly, i want more control of the colours/patterns for clothes; theres several shirts that have various colour and pattern combinations, but that dont have all the possible combinations as options.
another thing I'd like to see is the introduction of individual layers for tops and bottoms. instead of choosing a pre-made jacket/shirt combination, I want to be able to individually choose a jacket and a shirt. we can already do that with underwear, why cant we do it with the other layers? and then having the option of wearing a necklace over a top that currently would hide the necklace entirely would be great (same with bracelets).