I could see something intended as a streamlined ruleset that serves as a quick entry point to Rolemaster. E.g. Run Out The Guns or RMX. That seems useful. Of course, in some ways RMU is already more simplified than RMX! E.g. we got rid of the base spell attack table, the RR table, and simplified the moving maneuver table quite a bit. And we already have goal-based XP. But for example an RMU-X might only have the simple rounds version of combat, and maybe you could collapse most of the skills down to whole categories (perhaps picking one specialization, looking at the list actually I think that would work pretty well for a fast-moving easy-setup game).
A campaign path, either on its own or combined with something like the above, also seems promising.
Something that is a complete and fully fledged game based on RMU, not streamlined or simplified, but with enough changes that it merits packaging those core rules in it all over again, is essentially a larger project like Spacemaster. There's going to be a lot of repeated content, if you already have RMU you're essentially buying the core rules twice, and it will have to be either a really big product or have multiple books of its own. I think this is too early for something like that -- not that there could never be a place for it, but we need some material to support RMU first. At this stage, I think it would be better to have a setting/campaign book that works with A&CL, even if it doesn't necessarily work with, say, Spell Law (e.g. a historical setting with no magic). Best if it has material that can be used both in its own setting and adapted into other settings, which could be professions, spell lists, talents, creatures, etc. A lot of RM is about making your own things, but pieces that can be mixed and matched are pretty useful for that.
We'd also be interested in companions of various sorts as appeared in previous editions of course.