I agree that's what happend too Thorn from Spartan Ops he was to be on Team Osiris but from the back lash from Halo 4 Thorn was replaced by Buck from ODST! Don't get me wrong what they did bringing buck back was a smart choice. But too thrown away a character and leave his know abouts unknown ticks people off! At least finish what's started don't throw away something that's not done being told!
To be fair I believe its mentioned in the one of the sprint epsiodes he was replaced due to scheduling conflicts with the voice actor for thorne not because of his popularity. Which was reassuring to me because I really like thorne.
I strongly agree with A Sangheili's post above, they should both be incorporated. The didact and the warden both have alot of potentially and I found the warden to be intriguing mysterious. They hint at the wardens backstory in halo 5 but never actually devulge much, which is something I hope is covered in the next game. And also brought up in A Sangheili's post he can also have various bodies which gives you quite a few creative possibilities. For the didact I feel like hes yet to be depicted as the amazing warrior and brilliant strategist hes supposed to be, I find his story very compelling I just think they need to cut down on the monologues and show just how badass he can be in game.