Undersized, old punch drunk boxer with some name value who has 0 MMA experience and needs a pay day has always been the answer that I am amazed the UFC and MMA fans generally were/are too stupid to realize.they shoulda really milked it with him vs an aging legend or green power ranger guy. even if he loses, the build up/wonder would still be there.
You would attract curious spectators from the boxing world on top of those from the WWE world and have people genuinely intrigued in a UFC 1-style way.