The One Who Waits was an individual active in the Doctor's universe. Even the Toymaker, an incredibly powerful being with powers akin to a god, dared not challenge the One Who Waits; upon discovering the individual in hiding, the Toymaker ran, deciding that it was a game for someone else. In the end, The One Who Waits became the only individual the Toymaker dared not play any game against. Speaking with the Fourteenth Doctor sometime later, the Toymaker mentioned his decision to flee from the One Who Waits, with whom the Doctor was not familiar.

Well, all I would say...

We could see the One Who Waits in the Doctor Who future, which I would love, well, hope to see the 14th Doctor and 15th Doctor team up in the future to challenge the One Who Waits!