Using your criteria @Yorkie I'd go with this.
Critical
Fresh competition for Isak - Could Minteh become that? I'm not totally sure where he plays in terms of position and I admit that I have not watched him once. No idea about names outside of that personally.
An upgrade at RW - This is critical for me. Almirón and Murphy aren't good enough and don't get into any other sides' first XIs in the top half. On top of that, Murphy was the last recognised RW we signed. 7 years ago. Almirón arrived as a #10.
I think we'll have to look abroad for this due to the inflated fees that get quoted from PL clubs. Nico Williams is one I'd like to see us go for, Kubo is a good shout as well (Kubo would also bring extra marketing potential out in Japan, as seen with Mitoma at Brighton). Those 2 are the only names I can think of that would be massive upgrades, would like to see us get and I think would be attainable.
The Rafa Leao shout earlier; he's an outstanding player but I can't see us going for him or getting him even if we did. The guy is a widely renowned player across the continent, mainly plays on the left when we already have a few players that can play out there, don't think he would fit into our pressing style and would cost an absolute fortune. £80m+ at least in transfer fee and he'd want wages that'd make him our highest paid player off the bat. Would like to see it, like. But don't see it happening and fully understand why.
Desirable
Cover for Schar and Botman - Can't see us signing more than 3 players from abroad in the summer so tried to think of a PL one for this. Lloyd Kelly's name gets linked a lot, don't know a thing about him, so would trust in the club if he was to be the CB that gets brought in. We need another Botman essentially or someone that potentially could become another Botman due to Schär's age. Think we also lack pace in defence, if Kelly can bring that then great.
Cover for Bruno - Think we need cover for Joelinton in the summer more than Bruno imo. Is Tonali not more similar in terms of role and playstyle to Bruno than any of our other midfielders? Without Joelinton we lack physicality in midfield and is a big reason why we've been getting walked through in the middle of the park for almost 3 months. Is this Ederson from Atalanta the perfect one to bring in in that case? I don't watch Serie A so again don't know too much about him.
If possible
New left-back - Thought Hall was going to be this but he may well be just not rated by Howe. Do like that Kerkez at Bournemouth if we are to continue the press alleged Bournemouth obsession. Unsure if he'd leave so soon after signing for Bournemouth though.
More tactically-suited cover for Pope - Given our struggles without Pope this season, he can kick the ball into touch as much as he wants for me now. I think goalkeepers are an issue that no PL clubs outside of Liverpool, Man City and maybe Spurs have got nailed and when Liverpool and Man City signed their respective keepers, they were both at the time the most expensive GKs of all time. Committing that much to a GK when we have other more pressing issues in the squad isn't something I'd like to see us do. When we have the outfield nailed maybe. In terms of names, Mamardashvili from Valencia could be a shout but it's not something I can see us going for in the summer.
League. Remember being a bairn and buzzing that everyone's favourite real man™ Joey Barton punched him in 2010.