1d4 old school combat wheelchair variants:
1. Your combat wheelchair is a 4'x4' gelatinous cube, modified to have a seat. It's trained to not eat the person while they're sitting in it, and it's really nice that any monster that touches your chair gets paralyzed (save allowed), but there's a 20% chance per night that an uncontained wheelchair will try to eat its rider while they're sleeping.
2. Your combat wheelchair is a golem, composed of a random collection of household items like lamps, wagon wheels and whisks. There's a 1% cumulative chance per combat the golem goes berserk, and starts driving full speed around the area, cutting it very close when it comes to cliffs or doorways (natural 1 on d20 means a miss), and frequently spinning out (1 in 4 chance per round of being dazed).
3. Your combat wheelchair is a juggernaut. This it the D&D juggernaut, which means it's a chair-shaped lump of roughly carved stone, with an animal head (1d6: 1 horse, 2 dragon, 3 kobold, 4 wolf, 5 squid, 6 faceless human) and 1d6 protrusions that look like random animal parts such as hooves, claws, or beaks. It's slow to get up to speed (1" move + 1" move/round when moving in a straight line), but very hard to stop (current speed in inches on d6 for breaking down doors). The projections are also able to shoot out and strike creatures within 1" for 1d4 damage.
4. Your combat wheelchair is just a wheelchair. If you're extremely healthy and fit and have no encumbrance, you might be able to go up stairs, just like in all those Youtube videos.