The universal carrier was basically a taxi for infantry, wounded and light to medium equipment and almost all of the powers in ww2 had a variant of it but since i will talk only about the UKF i will only mention some variants that were made by the commonwealth forces and no modifications that require a redesign of the UC or by other factions (German and American variants)
Therefore, I would suggest a third upgrade for the UC which would be the 2" mortar variant (since the 3" mortar variant required a completely new chassis). It would give the Brits an early mobile mortar but since it is more mobile than a normal mortar crew it would need some tweaking and balancing even though it is a weaker mortar. I reckon the cost of upgrading to the mortar variant would be the same or a bit less then the WASP upgrade and would be unlocked after building the Platoon command post as to prevent having T0 mortar and mg.
I'm proposing this because all British platoons had a 2" mortar mostly used used for smoke shells (sometimes HE, flares and white phosphorus). Since the IS would be op if they could pop smoke baseline whenever they felt like it and the Royal engineers already have a ton of abilities, maybe the UC could have that role with the 2" mortar for delivering smoke shells, maybe a flare ability at vet1 (or not at all), and light HE barrage from a reasonable distance (less than other mortars since it is a small mortar).
The 3" mortar pit would still be superior in most ways to the UC so to not make it obsolete if this were ever added. This idea goes well with the British philosophy in the game of having to choose what units they make (AEC/BOFORS, comet/churchill), but in this case you can build both the mortar pit and upgrade the UC but they serve different roles. One is a powerful long range mortar which isn't mobile but has greater durability and a powerful punch, the other is a small mobile mortar with mostly support in mind (smoke, light HE barrage, flare on vet1?) and a shorter range than any other in game mortar. For defense it could have the front mounted Bren which would stay the same or have a decrease in accuracy or it wouldn't have it at all.
I have asked the reddit community about this and this concept was improved and tweaked to what they had to say, so it is already pretty much done except the numbers/balancing. I hope the devs see this and i hope that i could see this in the game one day.
I'd love to hear your thoughts on everything i stated and proposed here.