To keep it simple, when the merge happens I am afraid that there will be a new player imbalance between the factions. Why play Japan when they only have 1 set of campaign maps when Germany has 5? Japan and Russia are the only factions that will only have 1 possible opponent after the merge, but this problem could be addressed with a Japan vs Russia campaign.
While it would be cool to have a france or poland campaign, I think its important that all the current factions are evened out a little bit in terms of content. That way all of them are fun to play and there is less reason for matchmaking issues.
There is historical precedent for a Japan vs Russia campaign, the post below goes into great detail on what such a campaign could include.
