I'm thinking more on the lines of Feast, but with specific upgrade paths, i.e. buy A from supply, trash it to get B* (not in supply), trash that to get C* (not in supply). The description did mention many cards that "you can only get via specific other cards".
My speculation along these lines: of the 35 kingdom cards, only 25 are regular cards coming with randomizers. This would fit with usual numbers. Now 10 of these 25 cards are upgradable, and whenever one of this card is in the supply, the upgraded version comes also in the supply, but can be gained only in a very specific way.
How you can get the upgraded version depends on the card. An random example to explain what I mean (didn't try to balance the card)
Small town (cost 4, +2 action, +1 card).
During your clean-up phase, if you bought at least a victory card, trash this card and gain a Town
Town (cost 4*, +2 action, +2 cards)
Cannot be bought/gained in any other way than trashing a small town