Aaaaanyways, I think I got the silver bullet:
Lebensraum
Retain land after you raze cities
Citadels ignore tile ownership when placed
The problem with Lebensraum is that 2 tile radius citadels does so much more than allow you to "tunnel", as you say.
- 2 tile radius means you can fight entirely inside your own land in order to get to the next citadel spot you want to place. You never have to fight in enemy land, which means you can fight with permanent +10 HP healing advantage over your enemy, and no anti-warmonger fervor
- Having your next citadel placement be inside your land also means that your GGenerals don't need to enter enemy land to plant the next citadel. This means you don't need open borders via agreement or war to keep stealing tiles. You can do this while at peace, at no risk to your GGs
- 2 tile radius means you have a 5 tile wide front where you can plant your next citadel. That basically means blocking a Lebensraum civ with your own citadels to block placement is impossible, because they can just go around you
- 2 tile radius is immune to being blocked by citadels preemptively, because having tile steal that reaches further than the NoTwoAdjacency limit means you can just steal the citadels from the enemy.
If the tile radius remains at 1, but you can plant citadels in any land adjacent to your land, regardless of ownership, you can still create chains of citadels, but you have way more limitations than now:
- an opposing citadel can actually block you, and you can't just steal it
- You have to actually fight into an enemy's land in order to get your GG in position to plant the next citadel. For at least 1 tile you actually have to dislodge a civ on their own turf
- You have to have some sort of diplomatic contact with the civ you are stealing from in order to be able to path your GGeneral onto their land in the first place. You have to be at war or have open borders in order to culture bomb inside their land