HUGE retirement plans for mega-rich UFC fighter Chael Sonnen -- he tells TMZ, he wants to buy the WWE when he quits fighting ... a company valued at more than half a BILLION dollars.

And Chael's rep insists he's not joking, telling us, "It is 100% Chael's intent to buy the WWE." In fact, the rep says Chael is already meeting with bigwig friends in the investment world to enact his takeover.

Bad news though -- a rep for the WWE tells us the company ain't for sale ... at least not yet ... so Chael better be prepared to put a sweet offer on the table.

The rep adds, "We are a publicly traded company, so [Chael is] more than welcome to purchase shares of WWE stock and become one of our valued shareholders.”