Good things come to those who wait, and this is certainly one of them. This oft-requested feature will allow you to build certain buildings in the territory of members of your faction.
Indeed, as hinted at recently, Paradox are introducing allied construction to the game.
Unlike subject construction, this will utilize only your own factories, leaving your favored ally to continue with their own constructions as before. Paradox have limited the building types permitted for allied construction to Infrastructure, Railways, Airbases, and Radar, however, this setting is easily moddable for those interested in expanding their options. Developers initially allowed the allied construction of forts, but, well, the office MP proved why that was a terrible idea.
For balance reasons, constructions with levels dependent on, such as Radar, will be limited by the tech of the recipient country, not the builder.
What this means is quite obvious. Alliances will now be more important than ever, with far more room for economic and technological maneuvering.
Whether it can be called fortunately or unfortunately, you will not be able to forcibly boost your allies' technological level, should yours be higher, as that would be overpowered.
What you can do, though, is help your allies out, by taking on some of their production load, if your factories are up to par, allowing them to fight the good fight, while you work from the background. Or, at the very least, help them hold one front, while you deal with another one.
This mechanic will certainly bring far more options to the strategy and diplomacy table, and make us all realize why the Allies won WWII.