As for other attachments:
Lots of folks on here build grapples for their loader bucket. I haven't yet, but I've run into plenty of tasks just this past summer that could've used one. (moving downed branches/trees, etc)
I have a box blade for mine to grade the driveway. Didn't build it, bought a King Kutter 5' one at TSC, then painted it JD green and put a John Deere decal on the back. Did that partly to match the tractor, partly to clean up and cover all the paint chips and rust that it came witch. Seems pretty common for KK stuff.
I recently bought a KK middle buster to do primary tillage and fall tillage in our garden to help mellow out that soil. Works good. Would be easy to build a middle buster or subsoiler, not much too them. Biggest thing is having/getting the big chunk of steel for the backbone, I think the KK is 1" x 4" bar, was easier to buy the whole thing for me than tracking down a short chunk of big iron like that.