The admirals will also come into play later on as well. With Fujitora's failure to capture the SH's Akainu, might send in Ryokugyu to deal with SH's pirates and their new allies. Also we have yet to see an admiral or any marines for that matter play the role of main antagonist in the New World. I think Ryokugyu will make a great antagonist especially if his ideals are similar to Akianu's. Also we are not sure what the WG has up their sleeves in relation to SH's. Sure they could send the CP0 after the SH's, but is that their most powerful force they can send after them? Only time will tell.
Really cool idea to have an arc with luffy vs green bull.