Well, I would say "anyone currently at a different club but seen as having the raw material necessary to improve the strength of our squad" but I suspect that's an answer you already knew.
Everton, like every other Prem club, has a department staffed by people whose job it is to scout, analyse and evaluate players from every corner of the planet. I'd like to think they earn their keep, and would have shortlists for every conceivable player "type' the DOF might ask for.
If you're asking me to name players:
1) Kalvin Phillips. I get that he'd be pricey but he looks a worthwhile investment and he's younger than Allan and Doucouré.
2) Martin Ødegaard... maybe. I'd definitely keep an eye on how he performs for Arteta for the next 15 games.
3) Scott McTominay. Strikes me as industrious, reliable and consistent, which is why Utd would be nuts to let him go.
But we would be foolish to say that we cannot improve on what we currently have in the squad.