MTG Salvation

Transmogrify

3R - Exile target creature - that creature's controller reveals cards from the top of the library until they reveal a creature card and put in onto the battlefield. Then they shuffle the rest into their library

Another redundant piece if you are trying to make this type of deck work along with Oath of druids. The downside, you need a creature in play, but it also won't trigger for your opponents unlike Oath of Druids, and in some matchups can be a backup for when your opponent is playing big cheaty decks as well.
Obviously won't make the cut if you are not supporting that kind of deck, but in my experience, that deck has always been a lot of fun, just highly inconsistent and this could help make it more viable.

If I recall, Polymorph would sometimes be seen alongside Lab Rats and the like. I think you'd want some sort of spell-based token generation theme to manage this effect, which I understand to be only incidentally present in most cubes.

I'm in the process of trying to determine if I can make the Polymorph archetype a thing in the cube right now. It can be done now that we have enough good versions of the effect, but it's hard to get the right number of enablers into the cube...

It's a completely different deck than conventional Oath of Druids decks though.

A polymorph archetype would be interesting for sure. Unfortunately i dont think there is enough spell based token generation in red and blue for it to work well, and jeskai/grixis polymorph won't be drafted commonly enough to be worth taking up two 4cmc slots.

I made a detailed thread for it here: https://www.mtgsalvation.com/forums/the-game/the-cube-forum/cube-card-and-archetype/817737-the-polymorph-deck because I think it warrants some legitimate exploration.