My point is, unless this topic is purely limited to the context of siege balancing, I don't care. Keeps SHOULD be literally the worst place to go looking for fights as an aggressor. In the context of say a 5v5, if you reach your keep alive it should pretty much mean you are home free and clear and your enemy will get cut down by your guards if they try to fight you in-range of your walls. In the context of larger group fights the advantage should be less overwhelming but still a factor. I'd prefer mechanics like guards to mean more than gate games but the general problem you seem to be trying to address (keeps are too strong and shouldn't save you if you retreat to them) isn't something I agree is a problem.
If I can stick a keep right outside dungeon exists, that's a problem. If there are several hundred feet of nothing and/or basic woods and granum between my keep and the nearest resources of significant value, and a considerably longer distance to things like tephra and dungeons, I don't see how it's still a problem.
Also, my outpost suggestion would mean that if enemies hole up in their keep 24/7 you can sweep their outposts and they'll eventually lose their keeps. So if you want a fight with a particular group you just start flipping outposts until they address it or lose their keep. I think that's a line of thought far more worth pursing than nerfing keeps because we're afraid people will hide inside them.
TL : DR - If you are fighting next to someone's keep and didn't come fully prepared for an all-out-siege and you DON'T feel hugely disadvantaged, I would consider that a problem.