I considered doing a prehistoric set but there are several out there.
I have seen several people turn the restriction on Imperiosaur into the keyword Primal. However I feel that this is counterintuitive as the creature now has a keyword ability (restriction)which would negate it working with Muraganda Petroglyphs.
I came up with the idea of making Primal a supertype which would allow you to use it on any type of card with a Mana Cost whilst not interfering with the Petroglyphs interaction.
So you can have Instants, Sorceries, etc with Primal.

For those of you who don't know Primal means - Spend only mana produced by basic lands to cast ..... Which would move off the card and into the rules.This would allow lots of cool extended artwork cards like Nessian Courser in the set to give it that prehistoric feeling (Look - Its so old that it doesn't even have a text box) and that would be a feature of the pure Vanilla creatures in my set.

Mind you, that would mean that Alex would have to enable support for extended art cardframes with no text box and extended artboxes and I don't know if I want to do that to him }:-)

Hahahaha. As it happens, the upcoming update is going to bring a bunch of new card frames. They don't include extended art yet, but I have been thinking I do need to figure out a way to make the semitransparent text boxes on Planeswalkers work, and if I find a way to do that you'd probably get extended art frames for free :)