That would be really interesting if upgraded cards were totally allowed and there were also an auction: how high would be the cost of the veteran who has all cards upped?
A feared total imbalance of the game might even be avoided by this kind of market-dynamics:
If an upped veteran costs 40+ cards ... how many of these can you afford and still have a viable vault?
Also, upped vets could maybe bring only certain assets to the market to lower their price and get into the game. They could, e.g., offer
"a set of 30 upped cards, selectable by the recruiting master from the following cards: destroyer (max. 6), explosion (max. 4), minor phoenix (max. 6), death nymph (max. 2), Pulverizer (max. 3) ..."
or simply
"a set of 30 upped cards, selectable by the recruiting master from the slaves/mercenaries portfolio" (attach excel-file -> anybody has the link to that awesome "collection counting"-tool again? can't find it anymore
)