You can almost bet on there being C/U/R/E levels of rarity, just like the MMO and card game.
It would make more sense to make the commons all figures we would desire multiples of, like the Bog Monster, Murlocs, etc. Uncommons could be utlity heroes, and rares could be very good heroes. The epics should be extremely powerful, costly to play (in terms of honor or points or whatever) and dfficult, but not impossible to defeat.
Regarding collectability, pursuing and trading for rares is part of the fun, as long as things do not get out of hand. Collect what you feel comfortable getting and let the rest go.