I don't see Lesnar vs. Batista happening. For one thing, it'd probably be a pretty dull match. For another, there's nobody for the fans to cheer for as it's a futile effort to try to turn either of them babyface. The fans don't want Batista as a face, Brock Lesnar's the guy who ended the streak and everything about Paul Heyman screams heel manager. Lesnar's already beaten the streak, I don't really see what he gains from slaying The Animal.
Cesaro is nowhere near ready to be WWE World Heavyweight Champion as he needs to rebuild his status as a singles guy. If anything, Cesaro would benefit more from having a strong run with the IC title before possibly being elevated to the main event spot next year. As for Reigns winning the Rumble and Lesnar winning the title at the event, it's like the setting of Cena vs. Rock II all over again with the outcome already known before the match is even officially booked. I don't think WWE ought to rush things with Reigns as he's only 28 years old. They haven't rushed The Shield and it's worked out perfectly, so Reigns needs to be tested for a while when The Shield ultimately breaks up. He may very well blossom on his own, but it's also possible he's not able to rise to the occasion.
If a plan involves Lesnar going after the championship anytime between now and WrestleMania XXXI, I don't think WWE can wait almost a year before implementing it. Lesnar broke the streak and while that's something to generate momentum that could last through to next year, I personally have a feeling they're gonna use Lesnar for a title program for SummerSlam. If Bryan is still champion by that time, I think it'll be Bryan vs. Lesnar. It'd be Bryan's first of the Big Four as the champion, he's crazy over right now and it'd be an ideal opportunity for WWE to play the underdog card with Bryan. Now that Bryan's champ, portraying him as the underdog won't work in most situations against most guys, but it will against Brock Lesnar. Lesnar's the only wrestler to be NCAA Division II Heavyweight Champion, WWE Champion, UFC Heavyweight Champion and he's the one who killed The Undertaker's undefeated streak. If Bryan is still champ by SummerSlam and is booked strong, there's money to be made against Lesnar.
At the same time, there are options for Lesnar. While Lesnar could have a dominant title win at SummerSlam, I think there's one thing that many fans aren't considering: Brock Lesnar doesn't really give a shit about being champion. If there's any wrestler on the WWE roster who cares less about being champion than Lesnar, I haven't seen him. As long as Lesnar collects a huge paycheck when it's all said & done, you could dress him up in a pink tutu, call him Brandy Lesnar and job him out to AJ or Paige and I think he'd be okay with it. As long as he gets a big chunk of change, that's what matters. That's why I always think of Brock as the biggest mercenary in WWE history. Of course, all the wrestlers do it for money, but Lesnar just doesn't have that love or passion. As it looks like WWE is in the midst of a genuine youth movement, having Lesnar win the title looks like something of a step backwards to me.
I like the notion of Roman Reigns going against Lesnar next year at WrestleMania XXX and I think one way to go around it would be for both he & Lesnar to compete in the Royal Rumble match. Given his performance this year, you know that Reigns will be in the match next year. Maybe Reigns eliminates Lesnar and Lesnar, being the poor sport that he is, grabs a steel chair, heads back into the ring, proceeds to beat Reigns with it unmercifully, tosses him over the top rope to eliminate him, continues to wail on him with the chair for a bit before delivering an F5 either on the metal ramp, on top of the stage or on the announce table.
I simply don't see Lesnar winning the title. It was a mistake to do it with The Rock and Lesnar's appearances are even fewer than The Rock. As I said, it gives away the ending of his match at WM XXXI before it's even officially booked. However, I do think it might depend on Bryan's popularity. If he retains his massive popularity throughout the year, his segments on Raw continue to draw and ppv buys & WWE Network subscriptions are strong, I don't see them making the same mistake they did with The Rock, resulting in what amounts to a vanity title run with an ending you can see a mile off.