If a Monk's principle thing is punching people, then that needs to scale in some kind of level-appropriate manner. Right out of the gate, you need to be punching through wood/bricks. By mid-levels (6-10 or so), you need to be punching through fantastic materials like adamantine. Beyond that, "hits things harder" stops being relevant and you need to ascend into crazy-town. Punching the magic out of people, punching people so hard they straight-up die, punching people into next week (or at least next round), punching people out of the universe, etc. are all examples of things high-level punching dudes need to have.
As a kind of quick n' dirty list:
Level 1: Can punch through wood.
Level 3: Can punch through stone.
Level 5: Can punch through ordinary metals (iron/steel/whatever).
Level 7: Can punch through extraordinary metals (adamantine/orichalcum/whatever).
Level 9: Can punch through any matter (obdurium/unbreakium/whatever).
Level 11: Can punch through magic (force effects/dispel magic on hit).
Level 13: Can punch through life (finger of death/slay living).
Level 15: Can punch through space (plane shift).
Level 17: Can punch through time (reverse time stop, only the victim can be affected normally).
Level 19: Can punch through thought (the monk can simultaneously attack any number of targets he chooses within his line of sight. He can affect each target with any one of his punches, and he may use a different punch for each target).
Maybe slap a recharge on the higher-level abilities. Don't make them x/day or even x/encounter, just stick them all on a timer and say "You can only superpunch every 3 rounds" or something. It honestly wouldn't hurt anything if you didn't. They'd require regular melee attacks and you could only do one a round, it's not any worse than a Wizard tossing a SoD every round; in fact, it's actually shittier because he's doing it at range or as a touch attack and can often SoD multiple people.
There, that should basically cover your "being relevant at killing people" abilities. Along the way, you need things like flight though. So add in the following:
Level 2: Can walk on branches/rope.
Level 4: Can walk on water.
Level 6: Can walk on smoke/dust/clouds.
Level 8: Can walk on air.
Level 10: Can walk on will (flight w/ perfect maneuverability).
Level 12: Can walk through space (teleport).
Level 14: Can walk through death (resurrection).
Level 16: Can walk through dimensions (plane shift).
Level 18: Can walk through time (time stop).
Level 20: Can walk through reality (Monk leaves the game world and enters the real. You win D&D).
Tack on some mysticism-related stuff (detect magic, etc) or not as you choose, and give him some defensive skills, or just let him wear light armor. Through in whatever flavor you want, like Timeless Body and shit, but mostly that's just to fill out the class. The core stuff, namely moving and killing, can be pretty well covered by the above lists.
It certainly needs some tweaking and is by no means a full class, but those are the kinds of effects you need to remain relevant, and at roughly those levels.