Adding wall light switch for future ceiling fan light option
Number one thing, I'd mark the reserved wires in the switch box and the ceiling box so when you open things up in the future it's clear what's what.  I also write myself notes inside the cover plate with a sharpie.  

I'd connect the grounds and neutrals at both ends and cap the hots with wire nuts.  I'd not put a switch in the switchbox, I'd leave a blank there.  (Use Decora style switches.)  

If you ever put the light and separate switch in, it will be easy; if you never do, it will be safe; if you decide to do something different that you haven't even thought of now, it will be easy to see what's there and available.  

If you really want the maximum flexibility you could run conduit from the switch box to the fixture, depending on the framing and the layout it might not be all that hard with ENT (the blue stuff).