So I felt like playing MEP again, and spent some time during the weekend trying to get the first steps of a sandbox MEP version functional. Here's what I got:
-Removed hardcoded war of the ring events
-Changed start date to 30 years earlier for war of the ring scenario
-Removed faction restrictions for more sandbox-y experience
-Added some missing horselord CBs, including make tributary
-Made most princes of Gondor tributaries of the Steward/King by event when they take the throne.
-"Corrupted by Darkness rulers" have a 50% chance of becoming tributaries of mordor directly, per ruler, if easterling or southron group.
-Gave Tower of Ecthelion 1000 retinue size bonus to make up for this
-Oath of Eorl is represented as friendship between a new steward/king of gondor and a new king of Rohan
-Put Maglor in Himling, where he sits singing sad songs by the beach each day

All by his lonesome.
-It's a dirty fix, but made Sauron and the Nazgul orc religion so Mordor isnt spending all its time in wars of tyranny due to sauron revoking titles from those not of the umaiar religion. On the upside, this also makes Sauron much more aggressive, which is good since he is an evil overlord.
-Made Saruman religion uruk for similar gameplay reasons, also so that he gets access to the orkish/uruk prepared invasions
-White Council Mechanics (crude alliance system for the goodies)
-Once you get to 2000 prestige and are a guardian of middle earth, you are invited to join the White Council
-Now, if you get 100 relations with another guardian of middle-earth, you can send 500 gold to become friends with that person, in effect, creating an alliance
-For a real sandbox experience, you can now invite members of other races (elves, dwarves, halflings, if you are human and have the guardian of middle earth trait) to your court, which allows you to colonize wilderness provinces with those races as long as they remain in your court. This costs 500 gold and prestige.
This only supports the War of the Ring bookmark.
Releasing as-is as a submod, unlikely to be updated, and the team of course is most welcome to integrate anything they like into the proper version
Files:
http://www.filedropper.com/mepsandbox