My opinions for improving RMC:
The Combat Companion was a huge leap forward. Perhaps the best thing that's happened to RM in 20 years. Now, you just need to do the same thing for Character Law and Spell Law.
Spell Law:
1) A whole new perspective could be opened if an Arcane Companion presented a system for spells that is the casters' equivalent of the combat styles system that is presented in the Combat Companion. Lists become skills, casters able to cast up to the ranks the list is known. DP cost determined (as in combat styles) based on the options you include in your list skill. Features like increasing range, increasing damage or criticals, fumble reduction, base casting improvements, etc. are all add-ins that can be tweaked by type of list/spell.
2) Along with 1) above, get rid of some of the cheese in the spell lists. Bolt 2 has twice the range of Bolt 1. Seriously? That's what I get for 4-6 more caster levels, and that many more spell points?
3) Along with the above, get rid of some of the gaping holes in the lists, or standardize the holes so that it's commonly understood. If the cheese were eliminated, perhaps all lists could have common spell increments: levels 1-5 are filled, then additional spells are found at levels 7, 10, 12, 15, 20, 25, etc.
4) An option for 3) above would be to standardize everyone at 6 base lists, but allow the casters to use up to 4 more "sideboard" lists from open and closed to fill the holes in their base lists. You can fill a hole with a lower level or equal spell. I can't count the number of times players choose classes based on how full the lists look.
Character Law:
5) Smoothed stat distribution should be a given. The +5 increment jumps always seemed to me as a way to compete with that "other" game system, to provide an easy conversion for smaller stat ranges. RM players understand d100. No reason to dumb it down. Players, especially ones who manage to get stats into the 95+ range, ought to see a difference between 95 and 96, or 98 and 99.
6) The maneuver system as a core mechanic is fantastic, but whole sections should be dedicated on HOW to use it. As a brilliant and talented GM, I've learned over the years how to work it. For a newcomer, it looks like gobbledygook. I second the suggestions above that there should be a standardized mechanic for non-skill based maneuvers using stat bonuses. Again, I improvise, but that's with experience under my belt.
7) Better backgrounds and talents. Talent Law was a great book, despite the fact that it was written for RMSS.
I'm a skill minimalist, let me say. However, I recognize that there are folks who like to have more diversity, which is why RMSS/FRP appeal to some folks. It seems to me that a system of Skill Disciplines could be implemented, that would be the skill equivalent of the combat styles. Expand the basic skill list by no more than 5-10 skills to cover some additional core or aptitude skills, and then allow options to skill development that would increase the DP costs. Features like in-depth specialization, generalized knowledge, aptitude affinity (for use with non-standard maneuvers), etc.
Arms Law:
7) The one thing that I think was still missing from the Combat Companion was a good discussion and method for handling exceptional attack rolls that break the chart or rank limit. I have a way I plan to handle it (since it WILL come up), but that's me. It's always nice to have a common ground with other players for these types of discussions, and rule canon is the best way to get that.
