Let me put it this way...the fight with the big bad is not what is interesting. If you count the last two (?) missions and what happens after the credits (seriously, do NOT accidentally skip the cutscene after the credits or you will regret missing it, imo), I found it somewhat mind-blowing.
Again, it's not the final fight with the big bad that's exciting - it's the last two missions combined as an extended ending that matter. I'm still slightly floored by some of the things that happened, and I certainly think it leaves doors open for future sequels. I mean...there is clearly more going on - a lot more. I wish I could say something, but spoilers and all.