we should also allow custom sized boards, or custom rules, but I can see how that would be a nightmare to implement
Not really. I have already programmed something that goes a long way towards doing that, just for fun: the Variants Play-Test Applet. It allows you to specify board size, also for irregular sizes with the aid of 'holes', and put a great many pieces on those. The moves of which you can alter into almost anything (leapers, sliders, riders, hoppers, bent sliders, bouncing pieces, side-effect (multi-) captures. And as to rules it can handle multiple royals, the baring rule, winning by arrival at a square, automatic promotion/morphing by reaching a square or capturing a piece of a certain type.
And it even contains an AI that you can then play against.
I think we should add more chess variants, and chess fairy pieces, Shogi stuff, and allow creation of custom chess pieces or like, a service for premium users (or even free users) that allow sort of like, a mod library that allows us to download custom chess pieces with custom movements and themes and stuff. they have to be approved by a moderator of course because I want to play many variants of chess, or with variants of chess pieces that aren't on the website Such as the fire demon, the Emperor, The Igui, etc.
we should also allow custom sized boards, or custom rules, but I can see how that would be a nightmare to implement