I too, have beaten the campaign and ... am excited, relieved, and disappointed, all at the same time.
Excited: This entire experience was a great break from the norm. The RTS genre is wilted and getting a game, issues and all, like this was fantastic. The cutscenes were wonderful, even the in-game ones, especially the banter between Red Team. The implications of the post-credits cutscene are actually pretty enormous and make some sense.
Assuming the HW2 ring is the one we saw in Halo 5's LE, then it means the paths of the two groups HAVE to cross now.
But that leads me to a cautious optimism. We'll have to see where the HW2 DLC takes us storywise, but supposing Halo 6 actually has cross-over with Halo Wars 2, 343 will have to manage:
Cortana and the Created
Blue Team
Red Team
Fireteam Osiris
UNSC Infinity and co.
Spirit of Fire and co.
The Banished (again, assuming we don't reach some sort of conclusion there in the DLC)
Arbiter and co.
Halsey
Considering how ... up-and-down they've been with story-telling and narrative efficiency/effectiveness, I'm hella doubtful this can all come together. Yet, at the same time, I'm pretty hyped to see just if they can pull it off.
Merely the idea of Red and Blue team converging, the last Spartan-IIs left in the galaxy ... HNNNNG
In short: the possible narrative/gameplay opporunities opened up my Halo 6 x Halo Wars 2 has me incredibly anxious to see where 343 takes things. I have a feeling Halo 6 will be an end, of sorts, for Master Chief.
Relieved: I want to say that it went by really quickly, but I've played for 20 hours to beat the game on Heroic, no MP touched yet. It was pretty frustrating at times, especially Mission 6 (Hold the Line) and Mission 11 (The Halo). Those two were especially difficult, for me at least. I have to get better at reading tactical situations, responding to encounters with better insight into individual unit strengths/weaknesses (Hornet Storm FTW), and I have to step up my micro-game.
Disappointed: I'm a little disappointed with THAT cutscene as well, I thought the opening flashes and cuts really distorted the action. I really just wanted to see him in full clarity, destroying everyone.
And although I spent so much time actually playing the game, it still felt like it just ... ended. The abrupt nature of it threw me off. I think I was hoping for a climatic finish, like the original against the Arbiter, but on the other hand, I'm glad they didn't go that route and make things predicable. It's a mixed bag, IMO.
Also, the game on PC is buggy. The latest patch introduced the black screen loading issue (which is not all bad because the menu song is so good). The game CTD every time I exit a mission instead of taking me to the menu.
Anyways, sorry of the rant. I really enjoyed my time with the game. I'll get around to grinding out Legendary eventually.