I think if a new version is taken on it must divide the gap between RM2/RMC and RMSS/RMFRP users. Otherwise, in my opinion, there would be little point to the endeavor. Yes, it would need to TRY and bring in new players, but I think it would need to succeed in the short run before it had the chance to do that in the long run and that means appealing to the existing RM fan bases. There will never be a point where everyone is happy, but I think we can all agree it should attempt the best middle ground possible.
Too simple and you get HARP. Don't need another HARP. Too complex and you won't get RM2/RMC players to buy in and probably no significant new customer buy in.
Stats I don't really believe there's any major difference between averaged or added. Six of one, half dozen of another. So long as it's balanced I don't care either way. It will simply be based on how important stats should be... if the stats are more important, you add. If less, you average. I would lean towards add because I think stats should be important, but like i said, I am unconcerned either way.
Categories are an interesting topic. I'd like to see them go to simplify things, but doing that might have wider reaching effects than people realize. Changes in one part of a system can have unexpected results in areas you wouldn't normally consider. Umbrella skills are a good idea, but really it's just a shell game when it comes down to it. Having one "Armor" skill that you need to develop separately for Soft Leather, Rigid Leather, Chain and Plate is really no different than having them as their own separate skill apart for the perception of someone looking at the size of each skill list. But then, a good number of the 'criticisms' of Rolemaster are merely such perceptions.
I don't quite understand your point about Spell Lists, Body Development and Power Point Development. They must have different development costs. You don't want a Warrior and a Magician to have the same costs for all those things.
By development rates what are you referring to? Each individual skill, or the number of development points you get per level?
Something everyone needs to keep in mind also... we, here on this board, are not the average customer. Even with as 'small time' as RM is these days, we are a fraction of the customer base and we are, essentially, the fanatics. We are possibly the vocal minority and that needs to be considered by all of us.