Let's look at how vanilla creatures can be used real quick:
1. To expand the total card base. Done, with 1 creature + 1 dragon per element, roughly.
2. As damage dealers in mono-decks. Done, with the above.
3. To add more variety simplistically.
What could be different about a vanilla creature? The only thing than can be altered are health, attack, and cost. Unique combinations for this are limited, even more so with creative unique combinations.
Vanilla All attack no health is covered by spark
Vanilla all health no attack is near useless in elements, and would only possibly be good as catapult fodder.
Vanilla creatures as more cost efficient attackers, covered in a number of ways but mainly life's Cockatrice and Frog
Vanilla creatures as "sturdy" (out of Lightning range) attackers for a low cost, covered by Graviton Mercs
Vanilla creatures as a strong quanta sink, dragons.
Vanilla creatures for free, spark and the mostly vanilla Photon/RoL (vanilla before upping)
We also have pseudo-vanilla extremely cheap creatures (the living towers, as it were)
What does this leave? Well, we can make an all HP vanilla creature that's cheap for Gravity to have some fun with, that'd have to be able to compete with in-element Armagio/Titan and strong-synergy voodoo doll. We could make more 1 drop creatures with slightly better stats, but they couldn't outshine or be worse than Frog else they'd be criticized, and if they're roughly the same, then redundant. Besides, many decks usually have a surplus, barring certain rainbows or fighting a denial deck. We could go even further than dragons for even bigger quanta sinks, but this would essentially be another cycle where some outshine others and most are only somewhat useful; they could also boost the power of OTKs.
TLDR; The only things left are super saiyan 2 dragons and a rock.