Many pros and cons can be created from just adding Thel 'Vadamee or the Arbiter, to the rest of the future Halo games. One positive thing is that people will like to see some old characters return from the previous Halo games. However, one bad thing can come out of this. If he dies during Halo 5 or Halo 6, many people that liked the Arbiter will be upset that he died during the campaign. And the fans might get upset and possibly say something like 343 industries is killing the halo games or something. But, that all depends on 343 industries idea, and if it fits the story. Mostly, they want to make something new and adding the Arbiter from the previous games isn't probably their favorite choice, unless they decide to make a new Arbiter. That would be interesting.
Even though that the Arbiter is my favorite character from the Halo franchise, I think we should leave him alone, in my opinion. He has done enough from Halo 2 and Halo 3. Plus, if he does come back, I have a bad feeling that he will probably also be killed like what they did with Cortona. And it's sad to see my favorite characters die in any way, even if it was in an honorable or badass way. However, if they ever put him in Halo 5 or Halo 6, that would be fine by me. Plus I get to see what he looks like with new graphics and how he has changed over the course of time. Just as long as the Arbiter doesn't die, I'm fine with having him in the future Halo games.