Very interesting thread, Moxie. Thanks. I wish I could write a longer post...
Everyone is expecting me to answer Thiem (well, a few posters at least), and I surely include Thiem on the list, after all the question is who "can" (and I read it like "who has more than a puncher's chance of doing it").
Of course that, as of now, I put Nadal as the top favorite for RG. At the same time, I've been saying I don't think either he or Federer will ever win a major again (I do not root for that, it is just my opinion). The apparent contradiction is explained by this bit:
If you assume that tennis players decline in a slow and steady manner, than it would be quite illogical to bet against Nadal on RG. But I am assuming that the decline (in results) will be sharp. That at some point the big titles will just stop coming. This is obviously not that far fetched. Naturally, after this cutoff point is seen through, some near misses take place. That's part of the game.
So the "riskier" part of my proposition is that I already guessed that the cutoff actually was RG 2018.
Back to the point: If Nadal could show up in 2019 exactly the same way he showed up in 2018, he would be an overwhelming favorite. Against him, with a decent shot just Djokovic -- who would have history against him if they would meet in Chartrier -- and an with an outside shot Thiem, who is one of the few who know how to beat Nadal on clay. Apart from that, is science fiction.
However, 2019 is not 2018. Nadal will most likely be a tad worst than he was in 2018. He will arrive at RG much likely having won less, and less convincingly. At 2018, he was the #1, he was holding two majors. At 2019 he will be #2 or worst, will be holding precisely just the last RG title. His margins will be smaller. He will need to push his body to the limit.
Obviously, even a slightly inferior version of Nadal is an unbeatable adversary for 98% of the field at RG. So, the list of names of (realistic) possible Nadal beaters still is the same, Djokovic and Thiem. But for 2019 I include, as I would not in 2018, the "field". Some guy having a good day, and Nadal tired of the countless battles. At some point a warrior lowers his shield. Not necessarily you have Hector of Troy in front of you. Remember, the Red Baron was shot down by a guy on his first combat mission.