I’m excited to tell you that we’ve recently added Rengo in a way that it works in combination with any of the currently[1] available variants! Kindof like a variant modifier.
Our implementation uses the subvariant as a blackbox[2], and replaces each of its player slots by a team of multiple players. The team members alternate playing moves for the corresponding player slot in the subvariant in cyclic order.
For example you could play Rengo-Chess, which I guess means that we’re now also a chess variant site 
What I find exciting about it, is that (in theory) you can really use any subvariant, whether it has alternative sequential move order like Thue-Morse Go or parallel moves like Fractional Go. Even rengo is a possible subvariant to itself (though all that does is make the teams bigger). Internally we’ve called the concept “higher order variant”[3].
Full disclosure: I have not tested all possible subvariants, so 
hopefully nothing breaks!
Here’s a breakdown of the config form for creating a rengo game:
You can freely experiment with it in the demo view.
Here are some further ideas for adding “higher order variants” in future: Pie Rule, Miai Go, Democratic Go
and future
↩︎using only the base interface shared by all variants ↩︎
inspired by higher order functions thanks to @benjito ↩︎