- Location
- Washington, USA
First idea I had, mentioned quite a while back, was a rune-powered air dome large enough to cover the entire city. If they can't simply hop over the walls, more conventional sorts of defenses can then be focused on the resulting choke points. Seal-based barriers at gates (toggleable by low-level ninja guards so they can let approved traffic in), barrel just beyond the barrier with directional explosives underneath set to go off when it's used as a substitution target, etc. Since it's designed to address a well-known existential threat to Leaf as a whole, we're not necessarily limited to Goketsu resources for sorting out all the details.
The key part I'm proposing now, though, is making it Oro's problem. He's the one who worked with Akatsuki, he knows what they've got, how they think, which esoteric movement options they'd try first. If Hidden Leaf burned down, he'd be inconvenienced - since he keeps a lot of his stuff there, plus the Final Gift Program - so defending it could be worthwhile for him even if there were no direct research benefits.
Kakuzu didn't die of overlooking some esoteric loophole, or even really the specific physical trap. What got him killed was taking the hands-on approach with a whole city full of people who'd rather burn him alive than negotiate, which he had to do because Akatsuki still mostly act like a pack of missing-nin instead of having a proper chain of command for delegation and defense-in-depth. Given that Orochimaru wants to avoid making the same mistake, personal investment in Leaf's perimeter defenses is a big step in the right direction. Doesn't just directly make the Basement safer, it'll make the Hokage consider him more of an asset and less of a liability, worth rescuing rather than finishing off when-not-if he gets his own bad luck. Sharing details of the defense plan isn't some underhanded scheme to extract more shinies, it's a necessity of allowing said plan to function at all, by allowing other Leaf nin to productively interlock with it. Runes aren't portable anyway, static defense is the obvious niche for them.
And there will be research benefits, by forcing him to get creative about combining the raw power of whichever rune effects are easy to research with all the other stuff he's already got available. Maybe he'll run enough tests in the Basement to figure out that burns from the high-wattage runic equivalent of HOWS have a different depth profile at different wavelengths, and then crank it beyond the normal spectrum to create a cancer ray which cooks intruders right through their armor before they notice there's a threat, or "invisible flashbangs" which are harmless to anyone who lacks a cheaty-eyeball bloodline.
