Does Cilic, del Potro, or Ferrer make the Tennis Hall of Fame? Or will it just be the obvious 5 (Federer, Nadal, Djokovic, Murray, and Wawrinka) that make it from the Big 3/4 era?
To be honest, I don't think much about the Tennis Hall of Fame. I know some about the baseball HoF, which is a pretty big deal - but is a bit of a shitshow, in terms of consistency. That is, there are a lot of players in the baseball Hall who are worse than players without.
But to approach your question, I think you can start with the question: Who are the
worst players in the Tennis Hall of Fame and how do those guys you mention stack up against them?
Here's a list of every (male) tennis Hall of Famer with 3 or fewer Slams, who played at least half of their prime years in the Open Era (so not including guys like Fred Stolle, who is borderline but had his best years before):
Michael Chang
Lleyton Hewitt
Gorin Ivanisevic
Yevgeny Kafelnikov
Jan Kodes
Ilie Nastase
Yannick Noah
Patrick Rafter
Dennis Ralston
Tony Roche
Andy Roddick
Marat Safin
Stan Smith
Michael Stich
Slam winners not in the Hall:
2 Slams: Kriek, Bruguera
1 Slam: Orantes, Panatta, Edmondson, Teacher, Tanner, Gerulaitis, Cash, Muster, Gomez, Moya, Johansson, Costa, Korda, Krajicek, Gaudio, Ferrero.
The big outlier above is Dennis Ralston - the only Slamless player on the list. Ralston was a good player, but there are many better Slamless players - such as Tom Okker - who didn't make it. I assume his central role on the US Davis Cup team was key in his induction, but it seems to be an example of arbitrariness and who among voters you should hands with. EDIT: Oh, he won a bunch of doubles Slams, so I guess is in the "Frew McMillan" category.
Kriek won two of the weakest AOs, so I get that, but Bruguera? The Hall also seems to dislike specialists - a lot of those guys were strong clay players who were relatively weak elsewhere. Lots of Spanish players.
It is also weird that someone like Noah is on there but not Orantes, Tanner, Gerulaitis, Muster, etc.
Anyhow, Cilic and Del Potro don't stand out as among the most accomplished single Slam winners. Both won only a single Masters, though both won over 20 titles. But of Cilic's 21, 17 of the are ATP 250s. Del Potro is a bit more balanced, with only half of his 22 titles being 250s (he won a bunch of 500s). And of course there's some big differences: Cilic, in terms of his level as a tennis player, was really no better than guys like Tsonga, Berdych, Nishikori, etc. He just went Stanimal for one Slam. Del Potro, on the other hand, was a greater talent whose career was derailed by injury. It is hard not to wistfully imagine what his career would have looked like if he was healthier - I don't think it is impossible that he would have been comparable to Andy Murray, or at least been a close #5 in the Big Four era.
But it is hard imagining inducting either when you have players like Orantes, Gerulaitis, Bruguera and Muster not in.