We discussed this here
Steal-Improved Steal-Grand Theft Elements xD
Now on a more serious note, that would be quite interesting, but some cards can't really have this kind of upgrade. How would you upgrade a micro abomination further? Or an ash eather, or a minor phoenix. The upgrading choice seems quite interesting though.
I think the idea is a little different. This isn't a second level of upgrade, its an alternative upgrade.
E.g. For abom you could have the choice:
Option A)Upgrade to micro to get a 2|4 for 1
Option B)Upgrade to Greater to get an 8|8 for 6
... or maybe Upgrade to Dissonant Abomination to get a 5|5 that scrambles up to 3 quanta per attack
For ash eater you could get
Option A) Upgrade to brimstone eater to get a 2|1 that produces
for a cost of 1
Option B) Upgrade to Obsidian eater to get a 2|2 that bypasses shields 1
For reverse time:
Option A) Rewind return a creature for 1
Option B) Fast Forward - move the nearest copy of target creature to the top of your deck or shuffle a new copy in
As ripper mentioned, not every card would necessarily need an alternate upgrade. It could be limited to a handful of cards or even singular cards in each element.
I.e. Dragons could get alternate upgrades that give them breathe weapons instead of improved stats.