I think that Dominion's great expensive snowballers (first and foremost King's Court) could do with a debt cost between <8> and <10> I guess. It might just be a pet peeve of mine, but I think many King's Court games are simple decided the moment one player plays KC on a decent action while the other is still struggling to spike 7.
The challenge of hitting big costs early is interesting, but on such a snowballing utility card it leads to huge variance. (adding to the inherent variance of Throne room variants, which is in the higher part of the spectrun)
Debt is a great way to fix this "powerful and expensive engine piece problem", since it keeps the opportunity costs of an expensive card, without limiting its availability.
Another possible "fix" for KC I'd like to try is to make it cost 6 (or even 5?) with a Grand Market-like clause preventing you to buy it if you have a King's Court in play. It might make the crazy KC decks harder to build, but I personally would probably enjoy it more, including the challenge of finding a way to get several KCs in my deck.