it was kinda obvious after reverie ark when we find World has a king after that it final villain candidates were BB and Imu
also after 2 Year ts and specially recently what we saw from marines isn't worthy of final villain material
like what doffy said marines probably are part of war for one piece (war for pk throne) and like all other sides they will lose it (obviously shs will win it) also already fuji is doing more harm to marines and WG than helping them and soon will change side
swords are also decent marines and will change side with other good marines
also with Cross guilds paying for their heads, marines have way more enemy than before unlike pirates civilians aren't afraid of marines and can go close to them (they will run away if they see any pirates) and take them of guard and kill or injure them for money, now marines need to be careful around pretty much any one that isn't a marine or part of WGs military ranks, its even possible to see that for enough money some low rank marines turn against the higher ranks marines with big bounties that aren't super strong, if it happen they can't trust anyone