As far as the straight-forward side goes, it’s notable that Deadpool and Thanos have a history in Marvel Comics, particularly driven by the fact that they are in love with the same woman: death.
Similar to the cases with Wolverine and Bob, Agent Of Hydra, the case for wanting Thanos to have some kind of role in Deadpool 3 has two levels – one straight-forward, one more meta.
It’s a love triangle that would have to be partially manufactured in the sequel, as neither the Deadpool movies nor Thanos’ previous MCU adventures have previously established those romantic relationships, but it’s an affiliation that now has the capacity to be explored on the big screen.