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.
This is only tangentially related, but I really wish we had a tier thing going on.
Like, you open "let's discuss"-like threads for each card to discuss where to put them. We have a good time, we discuss, we argue, strong feelings, a bit of lightearthed drama, yay.
Then if somebody is not satisfied with Forge being in the B tier they may open threads to motion for a change in tier, and we have more discussion, more arguments, more strong feelings, more drama, more yay.
The tiers could look something like:
S: You extremely rarely don't want to make use of this, and when you win without using this you want to post a thread in the Games Report.
A: Kingdom definer - This thing shapes the kingdom around itself
B: Solid - This thing enables other stuff, counters other stuff, you may or may not want it but you must carefully consider its viability
C: Mediocre - It has its moments, but it's unwieldy, not impactful or just eclypsed by better things in its cathegory (if its cathegory is common enough that competition is common)
D: Depressing - Hardly ever worth pursuing. The kind of card that you risk not noticing when you look at a Kingdom.
With plus and minuses as appropriate.
I use a system like this in my personal ranking spreadsheet (yeah I have problems). Mine are:
5 - Centralizing cards that typically dictate the strategy on a kingdom (Cultist, Rebuild)
4 - Cards you almost never pass up but don't typically dictate the strategy (Hunting Party)
3 - Cards that are generally good options in most kingdoms (Village, Smithy)
2 - Cards that have key uses and don't get bought all the time (Baron)
1 - Cards that actively hurt your deck unless you have a plan (Thief, Rats)
The thing about my system is that you have cases like Thief vs Rats where Rats is clearly a better card, but I still stick it in the same tier because the system is about general use, not strict power. I don't think there is a perfect system that speaks to everything, personally, and when you add in aggregate scores from multiple people it gets even messier. If people are interested I could share my list, but I've already accounted for Empires, so I'll probably have to keep that off so you don't get hot playstester strategy tips. (lol)