That's a good question. Pre-TS I was 100% sure that M3 will fight the OG admiral trio, but by now the idea does not hold.
I still want Luffy vs Akainu at least. We haven't seen Kizaru use CoC, and twerkbull was no-diffed by Shanks Wi-Fi CoC, so it is highly likely that there are a selected few who actually have it among the marines. So if Luffy vs Akainu happens, it's either going to be epic CoC vs CoC, or Luffy just stomping Akainu like a Chihuahua puppy and I really am not a fan of the latter.
Also, from narrative standpoint, most marines are kinda like police, so once WG falls I would expect a coup within WG as well.
....on that note, Sanji vs Kuzan (who I also believe to have CoC) would be dope.