finish making AHII first before making WWI.....
A game like AHII might never be "finished". The subject matter is so huge that to game every aspect of it would take many many programmers decades to model and add graphics for everything.
Branching off makes sense from a business sense. People like Filth who would like a time frame that is almost exclusively ACM. With all the whining that goes on within this forum about squeakers, tool shedders, etc. I suspect that a WWI game would not have squeaker or tool shedders. It would have a more pure ACM environment though.
All that said, someone else's idea of adding biplanes to the EW planeset would be a good launching point. It would add to the WWII planeset/scenarios/EWA and it would give HTC a test ground for biplane modeling within the AH game design.
Suddenly I am think of a Spanish Civil War FSO... I-16, Early 109, Some bi-planes, HE111