I think it would be a pretty cool idea to actually play the story of the forerunners (basically the 3 forerunner books) as a warrior servant not specifically someone who was in the books but just a generic warrior.
We had a prequel with Reach granted its much more recent in terms of story and is about the spartans but with 343’s direction heavily focussed on forerunner I think it would be great to play as them in their own time line.
I see potential for this. Perhaps not for matchmaking, but for campaign. People liked how the halo 2 campaign involved the arbiter so this doesn’t seem so different. I wouldn’t have it as it’s own ops but i’d definitely consider it to be included as a mission in halo 5.
Recently, I was playing x-men legends (as a bit of a nostalgia trip) and what they did for the campaign was have ‘flashbacks’ in time. So in halo, it would show more of the timeline without having the responsibility of reintroducing them.
I don’t know if the idea of flashbacks would go down well with the community though. 
I wouldn’t mind a mission where you play as the Didact.
Perhaps its towards the end of the Flood war, and you and your Prometheans are fighting the Flood and the objective is to compose some Humans or something.
Flashback missions as the Didact would be nice, as well as a side campaign for the Arbiter.
I’ve been thinking about this a ton after seeing Didact in game. It would be really awesome. I just don’t think it should be mp. Maybe the next game could be about the Forerunners. It would be a good way for 343 to explain them to the video game crowd and we would get to play as BA Prometheans(pre and maybe even post Knight).