Maybe wishful thinking, but it's mostly Garcia's mentality that has regressed, so she has that advantage over the others... in their cases the mentality AND game have gone down the toilet. She's still fit (aside from the recurring back drama) and still has that serve. The problem is between the ears for her though, if she dumps her father and relaxes more on the court I can see her playing more respectably again. She did win a title, make another final (on 2-3 different surfaces) and led her country to the Fed Cup final this year. Signs of life are still (kind of) there.
In theory Muguruza has achieved the most and used to be able to scrape through matches on that 'champion aura' (plus RG and Monterrey are on the calendar to help her get wins), but having not won a match since the French Open is pretty damning, and like I said, her game has gone out the window as well as her mental edge. I see her remaining top 40-ish though.
Ostapenko can still get decent scalps due to her weaponry and she will still have random days where she's zoning, but I just don't think she can get by with that serve any more. It's one of the worst serves in the top 200.
Kasatkina just doesn't have much game to fall back on in the first place when everything else isn't working (whether it be mental strength, matchplay, determination, physical strength and fitness etc.). Even when she plays well she's under threat from stronger players, and her serve is super attackable too.
I'm thinking Muguruza too, sheerly because of her better slam pedigree but it’s not certain by any means simply because of how inconsistent she is through the year. It’s even worse this year, no win since RG.
Muguruza would be the most obvious answer here, but you never know. Also Garcia is better than she currently thinks she is, so if her head were in the right place she could absolutely get back. Kasatkina and Ostapenko are still very young so if they're both willing to put in the work they could also have a shot at reaching the top 10 again, probably Kasatkina more because I can totally see her improving her serve in the future. Ostapenko has to mature first, to really be willing to improve herself, and then we can talk.
Muguruza and Kasatkina have surely bottomed out a bit, and both of them have made recent changes to their team which might spur on a turnaround. Muguruza's slump has also partly been due to injuries, so if she gets healthy that could be key.
Ostapenko is still in crisis mode and shows no sign of turning it around, unless her team actually do what they've been saying for years now and fix her serve in the off-season.
Ostapenko and Muguruza have an advantage over Kasatkina in that even at their peak their top 10 ranking was based around a few peaks that came out of nowhere, and while they can't rely on that by any means it can still happen with the right draw. Kasatkina needs to rediscover a lot of confidence before she can start posting the consistent results she needs to for a comeback.
Garcia has neither hit rock bottom nor is in a crisis - she's merely resumed her brainless, limited level after fooling the world based on literally one random two-week stretch in her career. Even Ostapenko (four years younger) showed elite form more times than that. Garcia will never be in the top 10 again because she was always a fraudulent top 10 player when she was there in the first place.
I dont know what is going on in Kasatkina's mind. She had some very good results in 2018.
Muguruza has the ability. I didnt know how Garcia had that run late in 2017, but she usually isnt that steady.
I would say Muguruza most possible
Since everybody will dismiss her right out of the gate, I´ll just say that Kasatkina´s sharp decline is the most puzzling to me.
She ended 2018 on a high note with the Moscow title. Made deep runs at events like Wimbledon, RG, IW and Dubai. And contrary to popular believe she had some decent wins over players like Wozniacki, Stephens, Venus, Kerber, Konta, Muguruza, Radwanska, Barty, Wang and Sakkari.
In 2019 she has a 9-17 record with just four wins over top 100 players. Given the stage of her career and her style of game that´s a totally inexplicable trainwreck to me. #83 in the race is crazy. At worst she should have been Pavlying in 2019, hanging round #20-30, without bothering anybody.
Surprisingly, Caroline Garcia is still winning tournaments. Granted, other than her win over Donna, her run to that Nottingham title wasn't anything to write home about, the fact remains that she managed to string five wins together. Muguruza's Monterey run was more impressive in terms of who she beat. She can still beat top-flight players, with wins over Bertens and Svitolina this year, but she hasn't won a match in months now.
I actually think that because Kasatkina's game is more robust, it might be most difficult for her to get back on top. There are just many more parts that need to work.
I thinks they could all still do it, but the lack of focus is startling for all these women.
It looks so unlikely for any of them as they’re all struggling in their seemingly bottomless slumps, although I’d say Muguruza has the ability to turn her career around if she finds a good coach partnership and commits fully to the tour again.