Although I have no proof of what I'm going to say, it's just speculation... IMO if they've done well in British Forces, maybe they're doing another faction now (15 or 18 new units, commanders,their voices and a few sets of skins) what takes 6 months, without background music or maps, only 15/18 new units and I suppose they were good sales with the British, maybe and only maybe, we have this summer another faction and so they can delay AOE4 to next year 2019 and COH3 to 2020. I find it strange that we have had a patch and maps improvements and commuting only to keep the base player two years until 2020.
Either a new faction or single player pack akin to Ardennes Assult could be likely. Relic's 5 year plan and even the updated plan however, never called for a Single Player expansion thus AA was kind of out of the blue outside of the new faction. I feel like they dropped hints of a British themed one but I cant recall where.