Sounds like Balance Hell.
Are you even thinking about what you write?
Balance Hell is when you have single large matrix of playable factions.
For example you have mid-war germans and late-war germans and try to match them with same set of allied factions. Here's what problems I can think of problems with this design, that we've already saw in CoH2:
1) Mid-war units are UP (think of problem of SU having T-34 as best stock medium)
2) Late-war units are OP (problems with superheavies in large team games, e.g. JT, Ele. IMO these are mostly fixed by now btw.)
3) Authenticy problems as mid-war units perform close to late-war units (Why muh 8.8cm KwK does same damage as 57mm at gun?)
4) Constant flow of "give X unit to faction Y" requests, even though it doesn't belong there by different reasons.
Now if you split all your faction into separate groups by stage of war and match up only factions in the same group you can have both early and late stage of war units without breaking anything.
You can have your Italy facing Matildas and Crusaders, you can have Japan facing Stuarts and Lee WITHOUT having them to fight against IS-2, Pershing and Comet.
You can have many factions in your game, covering each stage, without having to balance them each against each - only in their separate group. And that's a lots of possibilities for DLCs, which should be interesting for a game company.