I like your groupings based on when players emerged so putting Goffin out of lostGen good. Players 30 and up right now are geriatric or borderline. Delpo and Cilic are LostGen in my mind as they both are still 29 years old. That gives a couple slams to that group and both fit the name with injuries and drug ban derailing their careers.
Its a little hard to group the bottom two when its a continuum with Thiem and Zverev 3.5 years apart, but Zverev really emerging not much than a year later than Thiem.
LostGen is a huge flop so far, but they should be winning in the current environment and Delpo and Cilic are getting results, though I think Cryic may be dropping with any luck. They must do something this year or next at the latest. Cilic could goat at Wimbledon and Delpo for the US Open.
ThiemGen really may just be Thiem.
Pouille seems to be stalling near the top ten. Kyrgios has been getting worse with injuries rather than better. Goffin needs to stay healthy and progress on serve just a bit more to start going deeper at slams, but really for him and Schwartzman it comes down to no serve, no slams.
NextGen will do well for sure, but only Zverev is the sure fire slam winner. I'm looking for matchup issues as these players face each other more and more. Tsitsipas just beating Thiem on clay is an ominous sign for Thiem's long term RG hopes.
For me the only player with a chance at double digit slams is Zverev out of this whole group. Players like Shapo and FAA are pure speculation at their age. I really like Thiem and the NextGen still a lot on paper. We really could be going into a very wide open era. I like the depth of talent in NextGen a ton as I just don't recall so many prospects coming up together since perhaps the techaided Americans in the late 80's and early 90's. Everything is cloulded right now when you have an 800 pound gorilla still lording over clay and Fed now dominant at two majors with the suddenly faster Auz Open. US Open hard court series may be where the initial breakthroughs come and of course clay also rans to Nadal. Let's face it, Thiem would have won a lot of big titles last year if Nadal had not been in his way.