XFL, ICOPRO, WBF, Linda McMahon Senate bids: just about every thing Vince McMahon is involved with outside of wrestling has been a failure. As was said, the last XFL had the perfect chance for success (aligned with NBC), and failed.

I'm too lazy right now to list the number of leagues that people thought would be a threat to the CFL and then died. The USFL had a great shot at success, but somebody always comes along to get greedy and screw things up.

Two docs cover these topics well
ESPN 30 for 30: This Was the XFL
ESPN 30 for 30: Small Potatoes: Who killed the USFL?