HoI isn't the kind of game that favours the kind of 'online-only, subscription' business model, and even less for something that could be free-to-play (Oh Gaijin Entertainment, I liked you so much more before you War Thundered....). The kind of model PDS has now, with base game plus DLC is probably about as good as it gets for these kind of games without the business model starting to intrude upon the gameplay. A subscription-only model would be right odd (subscription-only games, usually MMOs, are based on the idea of continued new amounts of content, and on larger servers required for upkeep - neither of which HoI should require - unless you're proposing that it's a HoI MMO where we all play an individual participant up to something like a battalion commander, and even then the constraints of MMO-style design would mean it would be impossible to have any semblance to history and be any fun (who wants to play the garrison in Scotland for five years?))
And a free-to-play model would be so poor for HoI's gameplay I don't know where to begin! I've seen so many promising gameplay ideas destroyed by free-to-play, and am yet to play a single free-to-play game that actually offers entertainment value anywhere near approaching pay upfront games (noting that I'm not a MOBA person) that I shudder to think at what it would do for the franchise. I'm not worried, mind, as I'm quite confident the devs/pubs aren't anywhere near that insane to destroy the value of their IP

.
Edit: Not trying to have a go OP, there's definitely nothing wrong with throwing ideas around

. This is just one I wouldn't personally get behind, and think it'd have a lot of problems (for another example, 1 month = 1 day, and it's online and with everyone so there's no pause or anything like that, so you lose the Battle of Stalingrad because it happened to coincide for when your family went out to Christmas lunch).