Planning Ahead; a D&D Campaign for the Future

I've been running Call of Cthulhu exclusively for months now, and love it.  I really feel good about the campaign I'm running and the additional single-shot games I'm running both at the store and at conventions.  But I also know that sooner or later I'm going to want a palate-cleanser from the unspeakable cosmic horror I'm hip-deep in.  And to tell the truth, because right now I'm not writing scenarios but running published ones, I'm not exercising my creativity *that* much on the gaming front right now.  So I've begun thinking about a D&D campaign for once "A Time to Harvest" wraps, probably mid-autumn.

Here's what I know I want:

• More "traditional" D&D than my last campaign; fewer rayguns, more Western European fantasy tropes and Arthuriana.  I'm likely to replace some of the D&D humanoids with critters like Sciapods, Blemmyes and the like from medieval manuscripts.  How is it Blemmyes never made it into any of the Monster Manuals?
• Sandbox adventuring.  "A Time to Harvest" is very plot-centric, and I'd like a break from leading the players from clue to clue and let them decide where they want to go and what they're going to break when they get there.  
• Rules-lighter mechanics.  Swords & Wizardry, B/X retroclones, and 5e are all possibilities here.  3.x/Pathfinder are not.  
• A sense that the world does not exist for the PCs, that there are constantly things going on in the setting that they aren't interacting with.  

Here's what I'm pretty sure I want:

• Early Modern tech levels; gunpowder and early firearms exist, though will not always be easy to get, with a matchlock musket being as "high tech" in this campaign as the X-Ray Pulse Rifle was in Devil's Canyon.  This has me somewhat leaning towards using the Lamentations of the Flame Princess rules.  Longbows, crossbows, and armored warriors will be much more common.  
• Strongly humanocentric setting.
• A solid mix of exploration/dungeoneering adventures and castle intrigue/stuff going on in town adventures.  A number of castles within the sandbox controlled by various wizards and warriors who feel very different from one another, so visiting Castle B isn't a cookie-cutter of having visited Castle A.  Likewise, different towns have different problems.  
• No reusing monsters; if there are orcs besieging Castle Blackstone, those are the only orcs in the sandbox.  There may be Chaotic-aligned, 1HD humanoids elsewhere in the sandbox, but they'll be, as mentioned above, Sciapods or something like that.  Mixed in with the Ogres, Harpies and Dragons will be the occasional weirder critter drawn from the Random Esoteric Creature Generator.
• A tracker akin to Chris Kutalik's Chaos Index tracking the ebb and flow and Magic and Mundanity in the sandbox.  

And here are some of the resources I intend to be using:

• 1E Oriental Adventures yearly/monthly/daily events charts to determine the regional events that the PCs can choose to ignore or involve themselves in.  
• Medieval Demographics Made Easy, by S. John Ross.
• Hexographer.
• OD&D Castle Occupancy Chart, combined with Jeff Rients' Random Castle Shenanigans chart.
• Justin Alexander's take on building a Hexcrawl.