I'm tipping something interesting and substantial, although if I was a dev and feeling mischievous, then I'd consider posting a 'fake' dev diary about each of the team's favourite colours or foods, and then a real one a couple of hours later

.
HOI4 started hot averaging about 12k players per day when it came out, plummeting to just under 5k in Dec. It rose back up in January with the DLC and averaged over 7k in the 3 months since. If it drops back under 5k again the peasants are revolting. Note the these numbers are no where near the daily figures for Stellaris or even EUVI (Steamcharts). Stellaris is in the 30k range while EUVI is around 13k daily. Suffice it to say they know they need a winner here.
Interesting stuff (although going to Steamcharts, I'm not sure where you're getting your figures, unless you're taking the latest data for Stellaris and EU4 reflecting the recently-released expansions)

. I likes me some stats, so had a look a bit closer (EU4
here and HoI4
here). Looking at the monthly average players:
- HoI4 launched stronger, 12,326 vs 4,486. Interestingly, HoI4 has never had fewer monthly average players than at EU4 at launch.
- EU4 maintained its launch strength better though - from launch to post-launch trough two months later, it fell by 29.8 per cent to 3,148. HoI4 fell by 59.6 per cent to 4,981 five months later. Four months after it's trough, EU4 had recovered by 32.0 per cent to 4,156, above it's launch level. Four months (March 2014) after HoI4's trough, the player numbers had hit 7174, a 44 per cent rise, but still well below the launch level.
- At no point since its release has EU4 had as many players (average monthly figure) as HoI4 did at launch (!) However, it has been slowly building its audience since it's post launch trough, and it's been above HoI4's current numbers (7174 in March) more times often than not since December 2015 (there are a few one-offs before then, but they're almost definitely spikes from sales or expansions, rather than ongoing trends) - over two years after its launch.
I'd say this suggests a few things about what's going on:
- HoI4 had much more pent-up demand - those launch figures are something else (Only Stellaris bettered it, at a monstrous 20,800* - data
here). However, it initially fell off pretty steeply, and that suggests that either there are a lot of people that were curious but GSGs weren't for them, and/or (I'd bet a bit of both here) that there was a fair disappointment at the initial state of the game.
- However, presumably because of the franchise's demand or strong interest in the time period, it has continued posting some pretty strong player numbers for a Paradox GSG. By way of comparison, Crusader Kings 2 hasn't had HoI4's current (March 2017) number of monthly active players
ever, and by some margin (data
here). Stellaris in March this year was just 5,042** (although it's currently smashing it on the strength of Utopia - expect April's figure to be
much higher - the peak in the last 24 hours was about 31,000). Other than spikes relating to sales or expansions, EU4's player numbers after they'd built up for three years were around HoI4's now.
Going forward, that suggests the dev team have a strong incentive to keep supporting the game, as it's already got a strong audience (in PDS terms), and the potential to bring back in earlier players who were disheartened with the earlier state of the game, as well as add new players, is likely to be pretty strong. In terms of future sales potential, there's a fair chance Stellaris is the only game with a chance to finish up on top of HoI4 long-term, unless the devs decide to infuse HoI4 with a strong theme of
Barbie Horse Adventures
(ie, if the devs keep working on improving the game, which they've given every indication is going to happen, things should keep getting better and better

).
* Monstrous for a GSG - no-one go bringing out Battlefield or Call of Duty numbers
.
** Stellaris fall post-launch was much steeper than HoI4s - it's trough was at 4,271, four months after launch.