It looks like Mata could be off and Sanchez and Lukaku aren't certain to be here either. With that in mind, an attacking selection of Martial, Rashford, Lingard, Greenwood and possibly Sancho isn't out of the question. Even if one of Lukaku or Sanchez are still here, that still leaves six players for three positions which means Mason would get more than enough opportunities. Especially so if we will be playing in the Europa League.
As for central midfield, both Garner and Gomes are great prospects. I'd like to see Garner serve as deputy to an experienced Defensive midfielder. However, I'm unsure if we can squeeze a sufficient amount of time out of Matic in the meantime. Gomes should be a solid squad option next season. He's more than capable of competing with Fred, McTominay and Pereira etc. He also offers something a bit different to them.
I'm a big fan of Tuanzebe and he should be competing for a first team place next season. O'Connor, Laird and Williams are all good enough to play a supporting role in the first team next season in my opinion.