MBLD. I think we might see 100 points MBLD attempts a few years from now, which is a rate of improvement I don't expect in any of the other events. I didn't investigate this very well though, this is just my intuition.
I agree with MBLD. I think big cubes will improve once hardware is improved, allowing faster, less accurate turning. I think all blind events are going to improve the most (mainly 4bld, 5bld, & mbld).
I believe 5BLD is probably farthest from its best possible result percentage-wise, even farther than multi. 5BLD really should be around 1.5x the time for 4BLD, considering the number of pieces that need to be solved, so the only reasons it's more than 2x is because:
1 Stanley probably doesn't practice it as much , and
2. There's still lots of room for better memo.