I think to provide amore dynamic look we should be able to put various rank on various uniforms. I would love to be able to put the TOS rank rings on my TNG movie uniform to have more of a Navy look to them. Also they need to fix the TWOK unifroms so that a LT is not running around with CPT rank on.
I'd like to see that but moreso I think costumes as a whole need to be completely opened up.
In terms of ranks i'd love to be able to use the Enterprise Terran ranks for my MACO toons, i currently use the SF Academy top for them since they closely resemble the combat uniforms in DS9, this ofcourse means i only get the neck pips (interestingly enough both cadet and standard rank pips, so that's a bit more open than most).
In a more open system i'd also like to be able to use the mercenary belt for them aswell, however this is currently gated behind the off duty system (of which i only have one slot unless i exploit a certain bug that comes up when purchasing new costume slots, so i must choose whether i want to use merc or formal sets even though i bought both) With the merc set i have the option to use the 7 of 9 hand but none of the glove options are available.
Klingonside i'd love to be able to use the merc set there too, maybe even mixing up the current single species costumes to be available for other KDF species.