How about making ultralisks the only unit faster off-creep than on-creep. Like a speed multiplier 1.3 off-creep.
I mean they have elephant legs anyways, don’t look like they work too well on creep. This also solve the problem that ultras always lag behind in a engagement. This also make them good units to pursuit marines, instead of getting kite to death by everything.
There is already an off-creep speed upgrade for ultralisks, which already has been buffed once, and already allows them to catch-up with stimmed bio. It’s called anabolic synthesis.
https://liquipedia.net/starcraft2/Ultralisk_(Legacy_of_the_Void)
As for adding a penalty to a zerg unit for being at home/on-creep. It’s as if you introduced a specific defender disadvantage, in a way. I don’t think it really makes sense. 
Yes, I know there is that upgrade, yet they still get kited to death usually without attacking even once (an easy buff to solve this is + 100HP of course).
But do you know why people don’t want to make ultras faster despite them not being fast enough. Because we have a mentality that they’d get way too fast on creep.
Beside, nobody make ultras for base defense (aren’t queens enough). Ultras are meant to attack and pursuit.
They need to add that charge ability that ultralisks have in co-op/campaign. Now THAT would be amazing.
Ultras have many problems. Which make them a bad unit… but the biggest problem is their clumsiness…
Ultras are the worst unit in the game, they barely can walk, by the time they reach the enemy they are almost dead or they still do not know what to do and start the ultra dance, until they die.
I think that would be a pit too strong since with decent mikro that would mean ultras are guaranteed to jump the backline since the burrow charge is more like a blink then a zealot charge. And it kinda goes against the ultras being a battering ram.
On the other hand the ability to walk over units and charge into the enemy zealot style would be a start. Maybe give units under the ultras an armour bonus to make it less punishing against AoE.
That’s basically blink though.
they just need an upgrade that makes their AI update from BW
There are some changes that sound pretty cool until you think carefully about how to implement them. For example, how the heck do you make zergline walk under ultralisks in the current model? Give them collosus legs?
I am pretty certain the LV ultra is way smarter than BW ultras. Much slower however.