In general, don't make anything you're not using. There are too many ideas and decisions which you'll only arrive at through practical use, which means that making without using will accumulate wasted work and required rework.
So don't make an engine without also making the game/content which will use it. And by extension, don't make your engine cross platform unless you're making a cross platform game.
That being said, as has been suggested above, you should make your design as portable as you can even if you don't port it. I suspect this will be good for design anyway, since isolating platform dependent code is a form of separating concerns.