#1 Feeder facilities
As we all know WWE has the Performance Center as their main location in which they train superstars of the future. Alternatively UFC has Ultimate Fighter, but they don't really have a facility that can match up to the WWEPC.
Once they do, the two companies should collaborate and create a feeder system in which they can send talent back and forth between the two.
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For example if a WWE talent is looking to improve their striking and foot work inside the ring, they could train at the UFC gym. Meanwhile if a UFC guy wants to work on his endurance and stamina, that could be something that the Performance Center could help with.
Hell, they could even just help each other out by recommending guys that they feel suits the other sport better. The possibilities are endless, should the companies get their heads together and decide to conjure up magic.