Combo wouldn't be a rush, mass hatch is stallbreak. Removing all of target creature from deck is too strong, encourages speedbow
If the deck built on it is strong enough to dominate the meta. But if so, the deck could be excluded from the effect.
^Hard to exclude a specific deck code wise. How would you define it? If it is defined as a specific exact set of cards then only a single deck is excluded. If you define it as a pattern of cards then players will start including that pattern just to get around this effect / encourage this effect.
Mass transforming all copies of a creature in a deck seems a little to strong in most cases. It will end up overly favoring rainbow decks which can cope with having random creatures in their deck. I don't really think that is a good idea...
If the effect could somehow ensure that the creatures that get created will be potentially castable then the effect would be less one sided toward rainbow quanta bases and this might be a little more viable.
I do think something along these lines could be interesting though. We now have enough different creatures available to each element that removing a single type can be imposed without crippling certain elements outright. However, it does need to be done carefully.
Unlike games like hearthstone or even MtG we have a very large number of base resource types and decks generally only have access to less than a third of them. This makes effects like this, which are typically quite doable in games like Hearthstone and MtG, much more complicated to balance here.
