Several Suggestions

A block to place in Wall/Floor Frames. This way if you accidently upgraded the frames you can still put blocks in them so they can serve as a proper wall and floor. This way you dont have to use c4 to fix a mistake or place doors.

Half foundation block. When you are close to higher ground the current blocks cant always lock in because they would be to far under the ground. With half blocks this can make your base bigger even if its only by a half foundation.

Auto adjust option on Floors. IT will automatically take the shape needed to fill the gap between floors. So if you placed walls on a Triangle foundation it can see the walls and know it has to place a triangle floor on top. Using this we could add more shapes like half foundation block or hexagon, octagon blocks etc.

Original builder of a block should always be able to destroy what he/she build. Maybe put a cost to it the longer you wait. This way building mistakes can be fixed without the need of expensive c4.