I believe Cross Guild may be an excuse for Oda to reuse old characters and congregate them into one place. It would be an opportunity to flesh out some characters who've had their backgrounds teased, but not fully developed. As a Guild, they can accept or deny certain jobs, unlike a typical Emperor crew where a subordinate must carry out their orders. It's also a way for Oda to provide Marines with Bounties, and an extra means to hype that organization up as we ramp up for the final saga. The Marines may very well be the ones Cross Guild conflicts with the most, considering their main operation is placing bounties on Marines, and Crocodile, the founder, convinced Mihawk to join him using his antagonistic history with the Marines.