I wouldn't be surprised if they're closely watching Destiny and thinking of making their own lite-MMO.
I wouldn't want this, and I hope that's not the direction they go. Destiny looks like it'll be a solid game, but there's only so much of that kind of game (mmo-lite structure) that I see myself being open to. To make a new line of Halo game in that structure would be a massive missed opportunity, I feel.
I want it to be a more straightforward singleplayer story that follows a clear script, that has a proper main character with a pre-defined identity of some sort, and also has full blown story cutscenes with key characters of significance that play meaningful roles in the story. Destiny is a very different style of game. I know it has story elements, but it's just not structured the way I would want from a major new game in the Halo Universe.
I'd prefer something more along the lines of Mass Effect, Uncharted, Tomb Raider, Star Wars 1313. Any of those possible blueprints are what I'd like to see for a new project line in the Halo universe.
If such a game as Halo Effect happened, it might overshadow the mainline games. An RPG of that scale would most likely be epic. Not against it happening, but I think any game off the mainline series would have to be less grand in scale.
This is precisely what I'd like to see. It wouldn't decrease the importance of the mainline Halo titles for me, but go big or go home is how I think they should be approaching this new project line. It should be such a major project that it deserves the right to stand next to the mainline Halo games. So good in fact that it should realistically get diehard Halo fans to take sides as to which they prefer more. Some may think that presents a risk to mainline Halo, but it would instead actually be to the greater benefit of the Halo universe/franchise as a whole.