I love the cube builder and draft simulation, and have thought of a few things that might be improvements!
The image view tab is nice, and I'm glad it is sorted by color. It would be helpful to include a secondary or even a tertiary sorting method. Being able to adjust sorting methods, it would also be nice to give the option to include events with the characters from a color category. Cubecobra lets you do this for their MTG cube builder as a reference.
Seeing the overall curve, curves by color/type, and type totals would also be helpful. It's cool that the table view is sorted by type within each color, but characters with multiple types throw this off. The downloaded CSV is good, but it'd be beneficial to see these on the page.
As I've been working on collecting the cards for my cube, I've thought it'd be nice to have a checklist to keep track. This may be a feature in the collection tool, but I haven't figured it out yet or seen a way to export the cube to my collection other than the deck you create after a draft (could be a skill issue on me).
This one is pretty specific, but for my cube, I like to draft the leaders separately, and it would be nice to be able to add them to a different section/exclude them from a draft. I just cloned the cube and deleted it from the clone, but it could be convenient.
Of course, these are all just suggestions. You have created a great tool that has been very helpful! The drafting tool seriously blew my friend and me away when we tried it.