After the initial comeback, I think Lesnar would be a big disappointment for 2 main reasons.
1. We've seen him fight for real. Shamrock came from the UFC in 97 when hardly anyone watched it. Most people had no idea who he was and had never seen him before. Therefor, his MMA wrestling gimmick could work. Lesnar has been one of the most famous athletes in the world for 2 years now. Millions of people have seen him fight and win in UFC main events. It would look ridiculous for him to start "fighting" like a professional wrestler after we have seen how he wins in the UFC. There would be no crossover audience from the UFC. The majority people that like the MMA usually don't like wrestling and think its stupid. Honestly, it probably wouldn't even drive up Wrestlemania buys that much. MMA fans might be interested in seeing Lesnar wrestle again once, but they probably aren't interested in paying 60 dollars for three and a half hours of stuff they don't care about when they can just see clips online.
2. Who would he wrestle? The Undertaker could work at Wrestlemania because of Takers size and MMA gimmick. It wouldn't be totally "believable" but it could work once. Lesnar has gone through the best heavy weight fighters in the world and dominated. Heath Harring, Frank Mir, Shane Carwin, and Randy Couture, who is one of the most respected fighters of all time. The single most important thing in wrestling is that the suspension of disbelief isn't broken. No one is going to buy Brock Lesnar against Edge, Randy Orton, Rey Mysterio, or almost anyone else in the WWE locker room. A match with Cena might work, but could you imagine him fighting the Miz or CM Punk? Lesnar against anyone but a super heavyweight would make people laugh. If the Miz got any offense in on Lesnar the audience would probably just start laughing. Thats the problem, eventually, a wrestler has to lose clean. With a few exceptions, there is no one in the WWE locker room that could realistically beat Brock Lesnar. If they do, it would look really "fake" and people would stop watching. In fact, WWE would probably lose fans to the UFC. Not to mention the way that ESPN and other outlets would mock Lesnar for loosing to someone half his size like Edge after a spear. It would be humiliating for Lesnar.
We have already kind of seen this before with Mike Tyson. At Wrestlemania 14 Tyson knocked out Shawn Michaels with one punch. Shawn Michaels was WWE champion at the time and arguably the best wrestler in the world. They had Tyson knock him out with one punch because fans would believe it. They knew how stupid it would look for Tyson to wrestle when he had the best knockout punch in boxing history. Same with Lesnar. How stupid would it look for Brock to go for a dropkick or a vertical suplex against someone like Randy Orton when everyone knows he could ground and pound and have the match won in under a minute. Brock Lesnar going back to the WWE wouldn't work in any way.