As the modder with the biggest active Imperator mod (at least to my knowledge/reckoning) I can say that its definitely not needed. The traffic just isn't there to justify the need for a subforum; with current player numbers, if all the messages I get on the GeS Discord, Steam Page, and Forum Thread were all put into the forum thread it would still be manageable (though a touch unwieldy). Contrast that with large mods on some other titles that NEED a whole thread just for bug reports or suggestions that is busily chugging along day in and day out and would swallow other discussion if confined to a single thread.
Sub-forums are (in my experience) something Paradox does for the REST of the modding community, so that visibility is maintained for other, smaller mods without being buried under a bunch of threads of the largest one(s). In the heyday of VeF and M&T (mods for EU), our combined active daily threads would have taken up a page and a half of the user mods forum if not confined to sub-forums. This just isn't even close to the case here; my single thread gets a post or two a day at the busiest.
So while I wouldn't pitch a fit at a stickied thread to improve visibility, I think sub-forums would be a waste of someone's time to create at this point. Maybe that will change in the future as things coalesce around a given mod or mods as the 'future of Imperator' but since I haven't even made a definite announcement on continuing work on my mod I think its a bit premature to stake a claim.