Actually, maybe doing both would be the best idea. A sourcebook that revolves around mass combat and stronghold building rules could include a sizeable portion devoted to an sample setting, kind of like GH is considered the baseline D&D world. Maybe all your sample strongholds and mass combat rules could use references to your Arthurian campaign setting book. They'd be complements to each other, and that could help you get past resistance from publishers about putting out another campaign setting.
I would think that you could put out the stronghold book (with Arthurian teasers) first, kind of like Sword & Sorcery did with the Creature Collection and Relics & Rituals. The stronghold book could help drum up interest for campaign setting's later publication. You could also put a few alternate rules in the stronghold book if you found room, but for me personally, I always appreciate d20 publishers who do not make their alternate rules supersede the Player's Handbook. Having "variants" is the way to go for alternate magic, et. al at this point.
Again just my two cents ,
C.I.D.
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Actually, great idea!! I really wanted to get my Avalon sourcebook, too. Thanks, man.