I, as well as many others, believe the flood are returning to the series in some form for Halo 5. In this scenario, it would only make sense for the Didact to join forces (reluctantly) with Humanity, and avoid repeating the mistakes of 100,000 years past.
If this were to happen, would you want at least a few missions where you could play as him, or would you rather stay MC the whole way? I personally would love to play as the Didact, not for half the game, but for say 2 or 3 of 10-12 missions, that would be perfect.
How would he control? I guess exactly like Chief, except faster, stronger, bigger etc. You could even get a cool armor ability that would act like a force push, using the constraint fields we’ve seen him use. Naturally the quantity of flood you would face would have to be 2-5 times higher than usual, which could make for some hilariously chaotic levels. Unique weapons could really spice things up as well.
The fun part, how could this fit with the story? Well maybe we find out he is alive at the end of Spartan ops, or a trailer for Halo 5. He could capture Majestic (or someone) and use them as hostages to speak with someone further up the chain of command. Then, he would alert humanity to what his sensors have picked up at the edge of the galaxy… the flood/precursors. Naturally you would play as MC to investigate, get a few missions in before humanity has to turn to the Didact for help.
His missions could be centered around something humanity cannot do (or something he would not let them do). For example, going to the greater Ark to find/build larger Halos. Why? To use as weapons to clear individual planets/systems and turn the tide of this new war. With a mission like that, the flood Gravemind would be sure to notice, and send one hell of a resistance.
So anyway, would you want this type of story? How would you like the Didact to appear in later games? Or would you rather he stay dead?