Then would ultra rares still be implemented (with a REALLY small chance)?
Yes, though either it would need to be EXTREMELY small or non-existant for the low-level packs, since the purpose of the lower packs is to help new players build their card base.
Under this paradigm, Nymphs would be out by default since they are non-grindable. Shards are currently grindable via arena, so yes, they could be in the upper level packs, albeit at a very low chance.
Then would ultra rares still be implemented (with a REALLY small chance)?
Ultra rares are not supposed to be grindable period.
Ultra-rares are grindable from arena. Therefore they are grindable...
However, if it is essential to ensure that they can only be gained by a player using the arena or pvp, there are ways to implement that as well.