An alternative for many posts would be to have no one in the office once the incumbent's term expires. For example, suppose no one wins a race for a seat in the House of Representatives: when the incumbent's term expires, that seat could simply be vacant, and could be deemed to have voted "abstain" for all votes during the term in which it is empty (or until a new special election is held to hopefully fill the seat).

(For an office like the President, there might have to be some other provision made.)

This would also handle the other case you mention:

> For more local elections, where running unopposed is not uncommon, there's also an interesting conundrum: should that candidate be rejected if they only receive a plurality, ie under 50%?

I would say yes: that counts as no one winning the election, so the seat remains vacant until a new special election is held (assuming someone does win that).

> What if literally no one else wants the job?

Then I would say the will of the people is that they prefer to have no one doing it, if they do not approve of anyone who wants it.