My main beef with the game is that there are several units which have no use or whatsoever. Here are some ideas for them:
Mothership: Right now it is a big waste of resources. No need to get back to the craziness that was the pre - patch mothership, but a suplemental hability wouldn't be out of order. My propositions:
a) Give it back the planetcracker but make it a researchable skill
b) Make the mothership act as a huge shield recharger, thus making it a great supporting unit like, well, a mothership
c) Impenetrable shield. A temporary skill that grants invulnerability to the mothership during a short period of time, and thus, preventing the "huge luminous practice target" effect of it being the only visible unit
Ultralisk: As some other forum user suggested before, a "charge" hability that would help him to get trought enemy lines and reach the enemy easily would work wonders with it.
Dark Templar: Its usefullness is extremely situational (basically harrassing zerg expos), and requiring an specific building only adds to its limitations, I have yet to see a player use them against me. My suggestions:
a) The most obvious one: do not require a completely different building in order to obtain them
b) Make them able to have the charge and / or the blink skill once your research said skills. That way they would rettain their harrasment role (not a frontline unit since none of their basic stats would have been upgraded) yet it would increase a lot their effectivity against humans and protoss too