There will be an odd site here or there which will let you submit artwork and such to create a Pokemon for fun, but you couldn't battle with it.
Smogon as a Create a Pokemon Project going on. However, this is a community type of thing. It's what everyone wants and votes.
We hosted our own Create a Mega Evolution, but this is no longer running. It will return eventually, but we don't know when.
Short answer:no, you can't sorry