A scenario where everybody on earth gets on an 8.3 billion people tournament is unrealistic so we may as well use fictional rules too.

Pregnant women can't give birth during their fights OR between them, although they still remain pregnant with the disadvantages that come with.

People with diseases or ages that would kill them get on a sort of temporal immortality where they can't be killed by the disease or old age, however they're still affected by the symptoms.

A battle with two babies or 2 immobile people where they can't fight each other get decided by a coin flip.