I'm play Fortnite rarely, and die a lot. Still, it's mouse/keyboard. Access to all your keybinds, better aiming, just... yeah. If you're really comfy with controller it'll work, but keyboard/mouse is king.
The main pros of kb/m is just that mouse-aiming is so much more accurate than just about anything else. The main pro of gamepad is that you don't have to move for any keypress really, and at least for me, it's easier to hit things like certain weapon slots, etc. on a gamepad vs. a keyboard, just because I'm more used to it. Fortnite doesn't seem quite as biased in favor of keyboard/mouse users as your standard shooters though, so I'd just basically say, if you wanna play on PC, use keyboard/mouse. If you wanna play on console, just use the gamepad. You're fine either way.
There's sooo many keys to bind in Fortnite I just don't think there's anything you could build that would be workable. (Aim, Fire, Use, Reload, Sprint, Jump, Weapon Slots, Build Slots, probably some I'm forgetting.)
Though in seriousness, something like a half-keyboard (like those left gamepad things, whatever they're called), re-organized a bit, with a built-in mousepad or trackball actually COULD be kinda cool.
Now in not seriousness, hold on firing up MSPaint.