There isn't much you can do about getting to 21mm on the x100, but you can get the TCL-x100 converter to get 50mm (equivalent). I got one recently, and I'm really enjoying it. It's quite large, and therefore is very visible in the optical viewfinder, but no problem if you use the EVF (obviously!🙄).
BTW, before buying it, I was concerned about the combination of the x100 and TCL looking a little weird (having seen photos of it, and being less-than-impressed), but IMO, it looks much better in reality than it does in any photograph - in fact, I'd go so far as to say that I think it enhances the appearance.
You can, of course, Google some photos of the combination, but I'd suggest a visit to your camera store to try one out, for real. 🙂