I like the idea of MK being high risk, high reward. Unlike now where it's little risk, high reward.
Actually, I was talking more like a high skill/high reward character, like Lucaryu. You'd have to be able to keep on the offensive, keep in short range, and keep combos going until they're finally damaged enough for the kill.
One thing that could help prevent the planking problem is a significant reduction in range. With a shorter Uair, that also doesn't hit so far to the sides, opponents should be better able break through it and punish MK for the generally poor choice of jumping off-stage.
This change would be good in general, since with attacks so quick he virtually has a persistent barrier of hitboxes surrounding him, making the barrier thinner would allow more characters a way through. Especially if it's transcendent, then opponents' hitboxes wouldn't have to be quite as disjointed in order to trade, or solely give, hits.
Also, would it be possible, desirable, and effective to implement a universal, in-game limit on how many times anyone can grab the ledge before touching the ground (sorta like the tether limit)?