thats something they def should not do. There are ways to include and or mention the faction to instill a sense of presence without using them outright in the launch campaign (shadows of reach does this). Dismissing or getting rid of them entirely only hurts the macro-level story of halo.
Yeah, if I’m honest I’d rather 343 just forget about Cortana and the Prometheans for Infinite, which would obviously solve the problem of having too many factions involved. I definitely think the most important thing is for them to get the Banished and Escharum right, but the Flood showing up would be a great addition IMO since we haven’t seen them for so long. But that’s just my opinion, I personally wasn’t a fan of the whole Cortana storyline in Halo 5 and didn’t enjoy fighting the Prometheans, but I’m sure that some people were and might like to see that storyline continued in Infinite too.
Yeah it's too late to just ignore that they exist. Cortana and her forces have been indirectly mentioned in a few books like Halo Envoy, and then directly appear in books like Bad Blood. They're too well integrated into the overall story to just be written off and never mentioned again. I didn't like H5's storyline either, and I don't particularly enjoy fighting Prometheans, but they exist now, so we have to address them.
I'm enjoying the way Shadows of Reach is handling the Created though. They're mentioned as a threat that the UNSC best not provoke. Could make for an interesting mechanic in Infinite. You can fight the Banished all day long, but you can't attract too much attention or Prometheans will show up that you'll have to deal with. Alternatively, if you're getting overwhelmed by the Banished, you could provoke a Promethean attack in order to distract the Banished forces.