Basically, every 30s you draw a card and you get to choose what effect would be the most desirable one. DPS would make the cut most of the time but the other two could prove usefull during progression. The other healer died and you got a Ewer? Could help.
Playing cards on DPS will always be optimal, which doesn't solve the current issue of omni-balance. You can't plan on RNG buffs and if a card needs a very niche situation to be useful, then it's not the greatest design. A healer needs to die (condition one) and you conveniently need to draw an Ewer (condition two). That's a bit too much.

I like your other suggestion better, mainly because it ties card to healing/mitigation. The main flaw of the current card system is that it is tacked on and has pretty much no connection to your healing kit.