I would disagree with your order, slotting Nadal ahead of Novak and maybe Andy. If they all make the QFs, then Rafa is definitely ahead of those two, assuming similar form to what we've been seeing.
But as for the pretenders, I think it is "real" to consider someone other than the Big Four winning, for several reasons:
1) Roger is 35, and the other three all have questions - Rafa's struggles on grass, Andy and Novak's current form
2) The "pack" is a bit closer than it was a year or two ago
3) Zverev's win at Rome opened the door from a hair to a crack
4) A fast court like Wimby = more potential for upsets, especially by hard-hitters/big servers
The bottom line: the chances of upset to any and all of the Big Four are the greatest they've ever been at a Slam, in my opinion. That said, one of them is still likely to win - but as a group it isn't like 95-99%, its like 90%.
As far as putting Nadal ahead of Andy and Novak? That's a coin flip, isn't it? I don't care what his clay court results were. The reality is - he hasn't been past the 4th round at Wimbeldon since 2011. Another reality check - check out the average rank of the guys he beat on clay this year - 34.12.
Monte Carlo - 45, 20, 41, 13, 24
Barcelona - 69, 66, 94, 84, 9
Madrid - 29, 20, 10, 2, 9
Rome - 73, 14, 7
Roland Garros - 45, 46, 64, 18, 21, 7, 3
I may be alone in this but I am not impressed. If he's playing even half-way decent he should be able to beat these guys
on clay in his sleep. On grass though? That's whole other ball game - and he knows it, too, or else he wouldn't have said his grass game isn't up to par and signed up for a grass court exhibition with Andy Murray this week.
Also, I must be missing something because I don't see how the "pack" is any closer now than it was 2 years ago. Dimitrov got hot at the start of the year - and has promptly fizzled. Stan can't get his crap together on a weekly basis and his French final was as much about Djokovic and Murray's decline and Federer's absence as it was anything he did on the court. Raonic was never very talented in the first place. Cilic is like a leaky roof that someone keeps plugging the hole but never reroofs. Nishikori just does not have the mental or physical gifts to stay with the top guys for 5 sets. Tsonga, Berdych, Monfils? The ship sailed for those guys 5 years ago. The young guns? Thiem overplays and has yet to learn how to peak for the big events or sting together back-to-back big wins. Kyrgios can't stay healthy - or interested. Zverev is still on an upwards learning curve in big matches, but obviously he and Kyrgios are the most promising of the lot.. And Pouille I'd like to throw a bone to, but just can't quite believe as a possible Slam winner. I like Khachanov's game, but man - he's a crap mover. I'd throw in DelPo, too, as part of the "pack" except he's never going to get back into the Top 10 unless he starts hitting over his backhand.
Bottom line - upsets will obviously happen, but in the end if one of the big 4 doesn't win Wimbledon - I'm gonna be downright flabbergasted. With Djokovic's decline everybody thought these other guys would see the opening and go for it. Instead - it's been a throwback to the days before Djokovic's hot streak - Federer and Nadal. If you're these other, younger, players on the tour at what point do you say,
"Why the hell can't I beat a 35, nearly 36 year-old who doesn't even play every week anymore?" and NOT find the answer, "
Oh yeah...because he's Roger Federer!" to be perfectly acceptable reason for your failure to beat him? John McEnroe got one thing right - why aren't these guys madder that they can't break through against the top 4 guys? Note I am NOT including Stan The Headcase in this discussion...