I think Kobi will be Luffy's oponent in the final fight. And even if he's not, he will be very relevant somehow. I wouldn't be surprised if he decided to change to Luffy's side in the very last moment.
He's been practically useless in the series, he became stronger, but was still nothing in Marineford battle. Until he started hearing all the voices because the haki was awaking inside. He's not a character I care about at all, but I think he hasn't said his last word yet.
And lets see what Robin discovers in the red Poneglyph. She's my favourite female character and very important, at least for me. I love her.
Useless? Thats a strong word for Kobi especially for someone who will undoubtedly end up as an Admiral in the reformed Marines during the epilogue. Shanks himself noted that because of Kobi interfering in the war that the course of history has more or less changed. He's not at the level of Luffy and all the battles surrounding him but he doesn't have to be to be useful. Not now anyway.
It will be because of guys like Kobi that the Marines will and up with less corruption or led by really questionable ideals of justice.
Outside from a few marines, they are not evil, a full battle against them makes no sense.
I think the last battle is against BB, maybe Akainu alone, but I don't think people like Fujitora,Coby and Smoker will be against Luffy in the end game.
As a whole no the Marines are not evil, the government as well. There are questionable people and dealings in both organizations but for the most part they have the best interest in mind of the world.
The thing is a Luffy VS the ENTIRE World Government will be a thing. White Beard foreshadowed as much and its simply because of One Piece. Guys like the 5 Elders and presumedly Sengoku know the true history and what One Piece means to the world as they know it, they know that releasing that information out to the world is dangerous and could cause some serious problems. Or would it?
Rayleigh told Robin straight up that when she discovers the truth she may come to a different conclusion, what ever that means. It tells us that maybe the government hiding the info isn't such a black and white issue. Guys like Garp and Sengoku probably whole heartedly believe that such info should not be allowed to exist in the world and fight to keep it under wraps. But then you have guys like Aokiji who were deeply affected by the actions he took to keep it all hidden. Not one of these 3 are bad by any means but if the truth is so great to be fought over it can and will happen. It doesn't hurt that Akainu is a super extremist that killed the main character's brother and has just about the worst ability one would want to fight.
What ever it all means, you can bet that Luffy and his gang are going have the entire world at each others necks. Basically doing what Roger and his crew could not do, fulfill the Will of D. It's the gods(Celestial Dragons) that pretty much own the world and their natural enemies(Ds) will rise up and half the world will be on Luffy's side and the other half will be on the Governments side.
Black Beard and his crew are probably the greatest villains the crew will face. I figure the battle for Raftel will happen between both crews. All this will happen before the World War because it has to. Doesn't mean there will be no high caliber opponents left for the Straw Hats. While not impossible I very much doubt the Admirals will go down one by one to Luffy as the story progresses. They can't get put in prison and they certainly won't die. We are going to get a White Beard mirror where Luffy takes Akainu, Sanji and Zoro get Kizaru and Fuji respectively and then Jinbe can show how far he's come by taking Green Bull.