Nope. Doubt this will get through. The very idea of allowing people to recruit in pretty much a second capital is not going to fly as long as we have the 'no tactical relocation of your capital' rule. Your idea pretty much goes straight against that rule.
Honestly, I dislike that rule. It's a rule based on intent, and that kind of rules are terrible. It's the kind of rule that means that a less rules-savy player who does a capital move for legit reasons but accidentally mentions the recruitment benefit can suffer the wrath of the titans, while a more rules-savy player who does a capital move for uniquely tactical reasons can pull it off without any sanctions. Titans aren't mind readers, and there have been at least a handful of cases where rulers were given the benefit of the doubt for otherwise extremely sketchy and convenient capital moves.
Same is to be said of strategic secession.
And realm mergers.
All of these policies... they are still there on paper, but there have been just so many cases of them happening without any divine sanction. Heck, friendly secessions used to be illegal, but it was conceded that we can't really fairly police attitudes. The rise of individualism and frenemies made all of these things pretty moot. Rules can evolve, and I think it would be fair to strike these out. With the game as it is today, and how it is likely to be for years to come, these behaviors are only problematic in the sense of unpredictable enforcement, of not knowing who will get away with what.
I mean, so what if people bring their capitals to the front? MORE WAR. Back when these rules were made, the distance from capital penalties were lesser, we had way more nobles, way more wars, colony takeovers were a fairly common thing (do they still even EXIST?). This ain't 2009 anymore.
More than being of the opinion of "you are right, this would break those rules, it's not workable", I'm more of the opinion of "you are right, this is incompatible with those rules, it's about time we scrap them". We've been struggling with "too much peace" for ages, all the meanwhile it's become harder and less rewarding to actually wage wars. Growth was penalized by shrinking the distance from capital limit, it's desincentivized by density mechanics, all on top of a declining player base.
We need more wars, not less. If it means people create mini-capitals at the border to more effectively go to war with their neighbors? Why the hell not.
That said, these archduchal capitals are not the same as moving the realm's capital. For example, if Astrum were to move its capital to Unterstrom, then it could immediately have access to 100% of its RCs to push the war against Swordfell (fictional example), and quickly massively renew their army within moments. If Unterstrom was made an archduchy, then only the nobles aligned to it could recruit there, and only from the RCs of regions that belong to it. This is, at most, a handful of nobles choosing from, at most, a handful of RCs. There may not even be all unit types, and many of these could suck. There would not be the same drastic effect of a strategic capital move, which' largest issue was the RAPID relocalization of power. After all, even with the no strategic capital move rule, Astrum would only need to take a few more regions east to justify "a more central capital". And, I mean, look at D'Hara, it's flopped its capital left and right while rarely having a really central one and never getting a warning.
It would imply, however, that Astrum would be able to expand into Swordfell. Not that handful of nobles with that handful of RCs isn't going to obliterate Swordfell, but it does add the possibility of taking a region or two. Astrum could grow. But... how is this bad? Given how we want more wars? The more Astrum expands, the more likely other realms are to react, as well. And these realms would also be able to prop up archduchies along the Astrum border to carve out some spaces there. Remember, please, that these are all fictional examples, I'm not a part of Astrum and I cannot say what they want to do or what they would do, I'm just creating fictional possibilities.
While at it, we could also scrap the realm merger ban. I mean, it used to do big controversies, but we've pretty much given up on it by now. Fact is, sometimes, a realm has no future. Either because it will collapse due to hostile realms, due to starvation, due to rogues, or plainly due to inactivity. The archduchy system would grant a framework for which to codify realm mergers. Rulers could surrender their realms to another, transforming it into an archduchy (make the process take long enough that if the ruler has gone rogue, players can rebel to make it abort, preferably with various mechanics favoring the rebels over loyalists).
If one is afraid of it being "overpowered", then bonuses can be given to realm capitals versus archduchal capitals, or building archduchal capitals can be more expensive rather than cheaper than regular capitals, or other such balancing mechanics. But while keeping cautious to not overly penalize successful realms. Because as it is, we have serious structural problems with this game. The continents are too sparsely populated, the maps have too many geographic barriers, and there are too many mechanics crushing both the means and the will to go to war. Archduchies would not really do anything direct for the first of these problems (unless they make things dynamic again, allowing player attraction growth), but it would certainly address the other two, by allowing realms to set up limited recruitment capacities beyond geographic barriers and by re-enabling the existence of vast realms.
