Just custom games and forge. This is the way I'd want them implemented if they were in Halo 5. Bots for all enemies as well as marines and spartans. You could also choose the difficulty: easy, normal, heroic, legendary, mythic. With spartans you could choose by matchmaking rank. Maybe bronze level or onyx level. Then you wouldn't need another player to Octagon with.
I think bots in custom games/forge could turn out to be a funny experiment and enable some nice game modes. Outside of that I don't see it as a good idea. There are so many issues with bots in multiplayer like balancing, programming an AI that works for any specific game mode/map, and so many more.