#1 Brock Lesnar
This would make the entire RAW Underground experiment a rousing success. Obviously, the biggest link between WWE and MMA will forever be Brock Lesnar. He was extremely successful in UFC, returned to the squared circle, and has become the biggest star in Vince McMahon's wacky world. Lesnar is being sorely missed right now.
The Beast will not return to WWE TV until live crowds are back and similarly, RAW Underground is likely here to stay only until WWE can run full arena shows once again. To optimize the impact of this ambitious concept, Lesnar could directly play a part in the conclusion of the RAW Underground storyline.
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Lesnar can return to WWE TV in a number of interesting ways, but an appearance on RAW Underground would certainly be as interesting, if not more so. Perhaps, there was a purpose to the Hurt Business showing up at Shane McMahon's "underground" club and dominating the place at the end of RAW. This might be the start of Bobby Lashley's build towards an eventual showdown with the Beast.
The Hurt Business could continue to dominate RAW Underground and take complete control of the operation, while Lashley constantly calls Lesnar out. He and Paul Heyman could make their WWE return by invading RAW Underground.
Bobby Lashley and Brock Lesnar would have a fight so bloody and violent, that RAW Underground is forced to shut down, ending the "underground" angle at the right time with a logical story in place. This will, hopefully, eventually culminate in a proper match between Lashley and Lesnar. A proper match in a ring with ropes. It makes complete sense.