In a couple of weeks or so, I've gotten well over 60 upgraded cards, which at 1,500 electrum apiece is about 90,000 worth of cards. How? I don't care about my win %, so I quit fights vs. any False God except Fire Queen, Miracle, Incarnate, and Chaos Lord, who seem to be the easiest to defeat regularly. Doing that, I win an upgraded card about once every 5 fights...usually one I didn't need, so I sold it for about 2/3 the cost...so doing some math here, I needed about 450-500 wins against those False Gods to get an entirely upgraded 60-card rainbow deck. If you play quickly, you can get 5 wins in an hour...about 90-100 hours of nothing but grinding....
So it might take 1-3 months to get one of these cards.
As I've said before, though: I didn't invent the grinding paradigm, I'm just working with it as an assumption.
Beyond that...the nature of the random shuffle/draw doesn't really lend itself to "fun" deck-building in PvP, because the most effective decks seem to be the simplest and therefore most consistent. E.g. poison decks and rapid mono-fire. Once you move beyond those two simple strategies, everything seems to revolve around two or three extremely powerful cards. Namely: Sundials and Hourglasses, so you can dig with reckless abandon through your deck and then out-spam your opponent. I haven't seen a decent PvP deck that wasn't either poison, monofire, or built around otyughs and sundials.
Personally, I like my permanent mutant alternative idea better. That would really increase the ability to build oddball (but effective) decks, because the monsters would be easy/easier to acquire. Unfortunately, I think it probably has an even greater potential to be game-breaking, because some of the mutants you can generate with Fallen Druids are exceptionally powerful.