There are a few possibilities for this one, but I hate to break it to you all, but Hulk Hogan isn't one of them.
Aside from the well known animosity between Austin and Hogan dating back to Austin's waning days in WCW, Hogan can't bump for Austin because of his health, and vice versa. No doubt, this is a match that would draw huge numbers as the dream match that never was. However, if Austin is going to return to the ring for one night only, I can't see him being willing to stink the joint up just for a big payday. If he is going to come back for a "last match" he will want it to be remembered favorably, and due to the issues of Austin's neck, and Hulk's aging knees and hips, there is too much room for error. Austin needs someone who can take care of him in the ring at Wrestlemainia 25, and I cannot forsee it happening with Hulk Hogan.
HBK- We have only seen Austin vs. HBK twice(King of the Ring '97, and WM 14) and both were good, solid matches, and the gut check by a near crippled Michaels at Mania 14 proved just how great he was. Could we possibly see these two men Main Event a second Wrestlemainia a decade later? Hell yes! Although Michaels has had knee issues as of late, he has more than stepped up to the plate as a prime time performer when the company needs him. He is in great shape, can still bump all over the arena like he could when he was 30, and Wrestlemainia will be taking place in Houston, TX. What better main event than a Texas sized one featuring the two top stars that ushered in the Attitude Era, who both happen to be from Texas.
HHH- I personally was never a fan of the matches that Austin and Hunter had together. The promos were always great, and HHH was at his heel best creating white hot heat, but for me the matches just never seemed to deliver. Dating back to Summerslam 99 there was friction between the two when Austin refused to drop the then WWF Championship to a newly emerging main event HHH. Whether it was that, or maybe the fact that their styles were just too similar, I just never felt satisfied after an Austin vs. HHH encounter. On the other hand, I think the promos leading up to a Wrestlemainia 25 match between the two would be worth the price of admission alone, and I'm sure they would come together to deliver a better than average match. Also, just like HBK, HHH can bump for Austin. Which if you can't tell, in my opinion is the overwhelming factor in who the Texas Rattlesnake will possibly face at a Wrestlemainia 25 return.
John Cena- For anyone looking for Steve Austin to "pass the torch" to John Cena as the mainstream star of the WWE, forget it. Cena's already held that torch for almost 4 years now, and I don't see anyone else coming around to take it from him. However, one major issue with an Austin vs. Cena match is how should it play out storyline wise? Before I go any further, let me point out that although Cena does get his boos, I believe someone in the WWE is sabotaging his face character on occassion. A prime example was at Wrestlemainia 24 when some local high school marching band came out on the entrance ramp, playing some generic marching band tune which then became John Cena's theme song. I was there standing on the rampway and when I realized it was his theme music when Cena came out, I turned to my brother and asked "Do they want him to get booed?" Personally I don't care if he is heel or face, and I don't hate the guy, but at that point I wasn't sure WWE knew what they wanted to do with him. That said, try that marching band stunt at Wrestlemainia 25, while pitting a supposedly babyface John Cena against arguably(aside from Hogan) the biggest star in the history of the sport, and proverbial hometown boy in his homestate of Texas. He will get booed out of the building by the guys, the girls, the little kids, and grandma alike, because for how over he is with his demographics(little kids and girls), he isn't Stone Cold who was and if he came back is over with everyone. If that's what WWE wants, then okay go for it. But as I said, I truly think top people are unsure of what they want to do with him sometimes.
Another issue with a match between them is Cena's in ring work. I don't think he's a bad worker, but I do think he gets a little sloppy at times, and I don't see Austin allowing himself to get turned upsidedown for an FU. Cena can bump for him, but Austin really can't bump for Cena's high impact moves all that well. 8 years ago, a feud between these two would have been gold. A heel Cena is just the cleancut kind of guy that fans would have loved to see Austin beat the hell out of, but that was a healthier Austin, and I'm not sure if WWE is willing to completely sacrifice Cena's face status to a one time match between he and the Texas Rattlesnake.
Undertaker- I don't see the Undertaker turning heel for this, and a win over a heel Austin(he isn't going to come back to work his last match as a heel anyway) wouldn't do anything for Taker either. This itself puts Taker out of the running, as I doubt they will try another babyface match like the one that bombed at Summerslam 98.
Orton- Maybe. If they want to pick up where he left off with the legend killer gimmick, Austin certainly fits the bill. Problem is for Orton, there really aren't too many legends left to kill after Austin, so why start that up if he can't run with it. Also for how good Orton is, he is also pretty boring on the mic at times, so I am not excited at the prospect of he and the verbally gifted Austin battling back and forth on the mic in the promos leading up to the match.
Edge- I think I would enjoy Edge vs. Austin more than Austin vs. Orton. The promos would be great, Edge can bump like crazy, he doesn't have too many high impact moves with room for error to injure Austin, and he would take good care of him in the ring. Of any of the younger main event talent out there, I truly think Edge would be the one.
Kennedy- The problem with a Kennedy and Austin match is that since he showed up on the scene, KK has been touted as the second coming of Steve Austin. Is Kennedy ready for an ultra high profile match like that? No. Will he be by Wresltmainia 25? Maybe. But in the same vein of HHH and Austin's styles being too similar, Kennedy is a virtual clone of a young Steve Austin, and I'm not sure how well they would mesh in the ring. Promos would be good, and both could go back and forth on the mic quite well with each other. I could even see Kennedy ribbing on how everyone compares him to Stunning Steve Austin, by coming out dressed as and immitating the Stunning Steve Gimmick leading up to the match. Actually, the more I think about it, the more I like it. And don't forget that Kennedy is supposedly Austin's protege, so that could play a part as well.
MVP- I'm hoping he will be in a title hunt by then, but I could also see him doing some great stuff with Austin leading up to Mainia. Just as KK has been touted as the next Austin, MVP is eerily reminiscent of a young Rock, and we all know how well Austin's character played of Rock's.
Best Chance of Wrestling Steve Austin at Wrestlemainia 25
1. HBK
2. Edge
3. Kennedy
4. HHH
5. John Cena
Austin versus Hogan
--As stated by Fierce above, the match would suck, but it would instantly sell tons of tickets. If Hogan says he has one more match outside of this, then Austin should go over clean. If they both want this to be their final match, then I say make it a draw, just out of respect for what both did for wrestling.
I normally agree with you NoFate, but not in this case. I love Hogan as much as anyone, but he refused to work a full program with HBK, and instead got the win in their only match at Summerslam. Because of the health of both Austin and Hogan, I don't see them even attempting a program more than a month leading up to Mainia, and they both hate each other anyway. Also, if there was a draw finish in a match this big, at a Wrestlemainia nontheless, the fans would riot. A draw or countout is okay as part of a storyline program which continues for months, or as a TV finish, but not at a Wrestlemainia. It's not like the two are going to shake hands, hug, and congratulate one another on a great technical match after the ref raises both of their hands and Lillian Garcia announces a draw. There is a very real dislike between the two men, and it is no secret to even the regular old marks in the crowd, let alone all of the internet wrestling community. If something like this goes down, the audience will feel more than cheated, and look for both men to be pelted with trash, and for the restraint barriers to wobble as security holds an angry mob back.