eras have been done before-- like in Stukoke's excellent World at War Mod.
the big problem was where to put everything and how to organize it. if we were serious about dividing things into era there could be as many as three or four times as many division tents as what we already had! some techs weren't even available early in the war. so we had to think about making multiple copies of each faction... even tech buildings.
the Corps HQ special op presents another complication. which Corps would you use? the op will only summon one actor every time. in Stukoke's mod he had the default special op Corps HQ limited to JUST early war tech. I understand why this was done-- because it penalizes players for losing. but it doesn't really make sense to be playing in the year 1944 in an RMC map and then have to revert to 1938 units because you lost your Corps. this becomes a major disaster in SP campaign games.
another problem is the WW2 Metagame. Stukoke had players spend extra money on late-war armies. this still didn't stop players from buying late war armies as early as 1941. I figured the only way to discourage this was to use stacking tech limitations. one year had to be researched before players could advance to the next year. if advancing one year in tech cost $200 players would have to think hard about whether they wanted those late war units at the risk of having fewer armies. one late-war army could defeat numerous early war armies.
the challenge was how do we add multiple time periods but have it work in both SP campaigns AND in WW2 mode?
the idea I came up with was a "Universal Corps HQ".
the Universal Corps HQ that would recruit era-based Corps HQ trucks. this building would have a year-based techs. players would start with 1938 techs and then have to invest each year in chronological order. it would use the same layout as heavy tanks being required before super-heavy tanks. you couldn't use 1943 units without first researching previous years.
the advantage is that we could leave the tech buildings largely unchanged. we could limit the units available through era-based division tents recruited from era-based Corps HQs. it would be confusing enough to have era-based division tents let alone era-based tech buildings.
it would also require minimal changes to the Metagame. we wouldn't need to add dozens of new period-based armies with different price tags. players could simply purchase the normal armies and know that they were already limited by yearly tech. it also prevents late-war units from being used in the Metagame too far ahead of schedule.
the Universal Corps would become the default special op building. players could summon a new Corps but wouldn't be forced to travel back in time. they would also still be limited to the tech they had previously researched. if they wanted to research their way into the future that would still be an option in RMCs.
the plan was to divide each faction into roughly three eras. (early, middle, and late)
all that being said... we decided that we weren't going to do eras until after we finished the basic version of the mod for the whole war. if we tried to implement it now it would probably push the project back about 6-8 months (at best). it's not just the major nations now. we currently have 20+ nations in this mod. if we do eras it would have to nearly every country.