As of right now I'd rank our needs in this order. Was hard to decide between 1 and 2 since 1 has the higher floor but 2 the higher ceiling.
1 - RW - Best case scenarios on current roster: Lukaku's promising play on the right leads to an impressive goalscoring season in that role (though he struggles in other facets) as he gives fullbacks trouble with his physicality and Fred comes good in the midfield allowing us to play more of a diamond in big games. Lingard has a solid year again. I've given up on Sanchez becoming our RW even though it still makes some sense to me.
Likeliest actual result: Lukaku has some good games at RW but shows he isn't a RW, Lingard struggles to impact games from out wide and with Fred just a decent midfielder we need to always play one. Mata struggles there again. Basically this year.
2 - RB - Best case scenarios on current roster: Dalot wins the job, has a breakout year at 20.5 years old, the same age Bellerin was entering his PFA team of the year season and 2 years older than Shaw was in his. Fullback is a young player's position and Dalot is highly rated, so it's possible.
Likeliest actual result: Dalot plays fairly well but doesn't outright win the job and Young falls apart, the position is comfortably our worst (Matic and our RWs and 1 of Bailly/Jones/Smalling/Tuanzebe starting is probably a higher floor than Young aged 34 and Dalot aged 19) and never a strength in good months.
3 - LCB - Best case scenarios on current roster: Bailly and Lindelof, or even conceivably Tuanzebe and Lindelof (or Bailly-Tuanzebe but that's really stretching it) form a partnership. Bailly will be 25 and he has the talent even if he has odd moments. He hasn't put it together and had a great season for us but I wouldn't bet against it happening.
Likeliest actual result: Bailly plays well enough but only 15-20 Prem starts as he gets hurt, is in and out of the lineup and everyone else has a massive issue that means they're a 6.5/10 for us, as Jones destabilizes us, Smalling can't pass, Tuanzebe is young, Rojo probably leaves but also see Jones.
4 - DM - Best case scenarios on current roster: Matic has a good season, 31 as a DM in a 3 man midfield isn't the end of the world, though he has a mare or two against some nippy #10s. This will be his 6th straight year getting 30+ league starts so he's been consistently reliable in that way. Also Fred cost a ton of money, did play well for Shaktar and if he comes good in year 2, which guys from other leagues have done in the Prem fairly often, Herrera can be another option as a 6 and transition there using his tackling and mobility like Gabi and many other 8s whose game was never really based around offensive creativity anyways have done in their late 20s
Likeliest actual result: Matic loses another step and it's a real problem and Fred doesn't impact games enough to let Herrera play as a 6, or Herrera struggles there. Matic has his worst season since returning to Chelsea and while it's hardly a disaster, it's definitely a weak point in the team.
Basically I think our needs should be ranked by how you rank the following to have a very strong season next year:
-Lukaku at RW
-Matic at DM or Fred at CM
-Bailly or Tuanzebe at CB
-Dalot at RB
Definitely think we have to address at least RW and one of the defence spots, and then it's not crazy to be optimistic on 1 of Bailly/Dalot and then 1 of Matic/Fred having the kind of seasons needed to compete for the league as starters.