This "safety rope" already exists (with Hobbiton and Imladris); the whole thing might work with the WorldwideAncillaryExists condition:
Code:
Trigger one_ring_battletrigger
WhenToTest PostBattle
Condition WonBattle
and IsGeneral
and not I_EventCounter ring_destroyed > 0
and not I_WorldwideAncillaryExists one_ring
AcquireAncillary one_ring chance 100
I've added the possibility to stay in the Minas Morgul or the Black Gate regions for four turns to pass the Ring to Sauron, as ... one does not simply walk into Mordor, even if he is Ugluk 
And, obviously, this is what happens when Frodo stays in Gorgoroth for 5 turns, and has the Ring :
Code:
monitor_conditions I_EventCounter ring_destroyed = 1
console_command remove_ancillary Frodo one_ring
console_command remove_ancillary Frodo gollum
kill_character nazgula
kill_character nazgulb
kill_character nazgulc
kill_character nazguld
kill_character nazgule
kill_character nazgulf
kill_character nazgulg
kill_character nazgulh
kill_character nazguli
kill_character Sauron
console_command surrender_regions england
terminate_monitor
end_monitor
The immortal Nazgul get a condition "and not I_CompareCounter ring_destroyed = 1"
Is there a possibility to remove a building ? ...I would like to destroy the Barad Dur of course, by destroying the Ring. I haven't found any console command to do so.
I would also like to enable the other evil factions to help Sauron to recover the Ring. Actually the postbattle trigger might give the 100 chance only to the Mordor generals, a bit lower to Isengard, etc. There might be a general "ring bearer" spawn with the other evil factions, to try to bring the Ring to Mordor (only this unique servant would be able to do so). And only Frodo can destroy the Ring for the good factions. Should he join the other non-AI good factions to offer them this feature ? In the same way Aragorn joins Gondor... that means that when playing for Gondor, you might get Frodo spawn somewhere in Ithilien in 3019... but the same thing might work for Rohan, for the Elves.
I am not really sure how this might be managed by the AI... perhaps Frodo not existing, and a set of chosen regions taken, might do the work.
I also think this is not bad:
Code:
Ancillary gollum
Type Follower
Transferable 0
Image ZGolum.tga
ExcludedAncillaries gollum, sauron_mace, staff_saruman
Description gollum_desc
EffectsDescription gollum_effects_desc
Effect LocalPopularity -1
Effect Unrest 2
Effect Piety 1
Effect MovementPoints 5