I think ranking the cards by cost has always been a bit suspect, and now that there's debt, that makes it even less relevant. I suspect the original reason for that ranking system was to give players a sense of what cards they should usually look for first when they have an $X hand. Being able to identify the power level of cards in a kingdom is one step in evaluating what strategy to play. But it's a vanishingly small part of planning.
My suggestion would be a card database that lists a power level for the cards, from 1-10, with decimals (since it would be an average from a voting process.) That power level takes the cost into account. Expand would be one of the biggest power cards in Dominion at $3. It costs $7 for a reason. Chapel would be really marginal at $6 (I'd rather have a Count at that point.) The cost shouldn't be considered a separate thing from the rank.
The card database could be sorted, high/low or low/high, could be sorted by cost if that's what you're into, or even by set. There could also be checkboxes for card types and for card categories (draw, +actions, sifters, trashers, junkers, etc.) This starts seeming like a useful tool. It's not quite as useful as the Magic card database, which is set up much the same way, because you're not building decks from a collection, but still, a helpful study guide.
I realize that this might cause issues like Smithy having a higher power ranking than Nobles because of its cost. You usually wouldn't buy Smithy on $6 with Nobles in the kingdom (I said "usually" to avoid edge-case posts. I can think of a dozen.) But look at it this way: When you look at a kingdom with Smithy and Nobles on the board, the Smithy should jump out at you more than the Nobles, because it tells an important story about how quickly players are going to be drawing bunches of cards.
And that's really what it comes down to. When you look at a kingdom, which cards/events/landmarks should jump out at you as "pay-extra-attention-to-me" items due to their power level. Yes, sometimes there are a lot of different strategies in a kingdom and you should look for synergies and subtlety, and sometimes there are Goons.