It's been a little while, but I've had another Balduvian Trading Post-esque idea that I would like to share.
There are a number of card templates that more-or-less scale linearly with cost. For example:


Why not lean into that and have a cube where you spend one cube slot on any number of these cards, and have the drafter pick which one they want to use? In other words, the draft includes X spells where the value of X is decided while drafting instead of mid-game.
The real question, of course, is whether or not there are enough of these templates to work with...