it's not done by a long shot, but I'm working on making a pony figure that's compatible with Lego's Character & Creature Building System

once the base figure is done I'll start work on addon parts, manes & tails, horns, wings, even clothes or armour.
the finished project will be ending up on Thingiverse, since I'm doing this purely as a hobby and don't really have any wish to make money off of it. Anyone interested enough to print it out would be required to sand the parts smooth, add smaller details, and then paint all the parts to the desired colours

You should add that to Lego Ideas. I really like it.

6833511
Go for it

6833676
Their a no custom parts rule on that sadly, also MLP is owned by Hasbro a toy company which is decidedly not lego.