The suggestion to decrease the stability loss of T2 and T3 sounds like a good evolution of the OP’s intial proposal.
Personally I know it myself how hard it is to encapsule a circle big enough it would be able to hold a maproom inside of it so it would be a welcome change if it would mean something to build higher Tier, but just more HP.
The downside I see is the possibility to plan out buildings, sure you would still be able to do the T1, mabe floor by floor, but you would never exactly know how many pillars you can remove in your final T3 build.
Wedges having more stability would be really appreciated by me since I think the way it is now it limits you when you are building a structure which is not a square, and that could not be in anyones interest I suppose.
About what @Shadoza said, there is a suggestion making wallpapers/skins for walls, this would leave you the choice how to skin your base but not lossing HP, check it out: