Does anyone know if the Monopoly game has a wheelchair game piece?
Maybe they made a X-Men version with Professor X's chair. Genuinely curious.
As for D&D, I'd personally dislike a plain old magical chair unless it was used by a villain, but those are usually powerful thrones with awesome powers. SKT has such a throne - can't recall now if it has the ability to fly.
For players or allied NPCs, I'd much more be in favour of a team that assisted their disabled companion in areas of difficult terrain or to mount riding beasts. They could even have a retainer or loved one specific for this duty, ideally like how Bran was treated in GoT. I see this playing out in similar vain to how halflings sometimes need assistance from their adventuring colleagues in water-clogged areas that are 4-5 feet deep. It is grittier style of play which is a personal preference and increases the difficulty by adding an extra layer of complication for both the exploration and combat pillar.
I also like some of the ideas expressed upthread of the exotic modes of transportation (tentacled contraptions, spider-legs or biological rides such as giant snails etc).
EDIT: Specific tools, materials and artificer services could also play an important role with such a character in a group. And perhaps a spare Floating Disk spell in situations of emergency. Overland travel by carriage or wagon could feature more. There are something interesting things one could incorporate with such a character.