Not having to recreate designs would be neat, but there's that little problem of character skills & weapon availability.
If you don't have the weapons available, or the hull mods unlocked, or the ordnance point boosting techs, a variant design that relies upon them is useless.
That said, saved variant designs outside of the campaign (where weapon availability & character skills are not a consideration)
ARE shared between missions.
Being able to share these via the Steam Workshop would be a neat feature.
It'd also be nice if there was forum support for them; export them as a simple String (hex encoded) to the clipboard, paste them into the forums in an appropriate tag & have the forum generate some nice html that presents it like the in-game ship design screen.
[variant]myCoolVarient:34B4E2F2DA49898FFE3[/variant]
Tbh we could do this ourselves with a fairly simple game mod + web host to generate an image based upon the String.