I agree that it would make things much simpler for newbies to get started if they didn't need to worry about which cards to have and about renewing the rentals. I believe that renting cards for a fixed price would go a big way for many. They could be rented per splinter and per level. For example: lvl 2 summoner with the according developed cards of the same splinter for that level.
By having such a paket players could with one transaction rent for a fixed amount of time and be sure that they wouldn't lose the rental at EOS. We definitely have to think about it :-)