Regarding GameBryo there is a reason that Trion and Bethesda use a heavily modified version of GameBryo. I know that Trion only uses some of the tools (which were to get them up and going while building their own engine) and Zenimax Online is not using it for their MMO. The architecture of this engine is old and stale and the main reason Bethesda stayed on it for single player games is because they built on it over many years and games. GameBryo is very similar to the Wild Magic engine in architecture although it has evolved slightly over the years.
I'm not sure about FPS MMORPGs honestly, but here are the "only" engines currently for sale(without private connections) that I would consider using for an MMORPG.
Regarding UE2.5/3 it has been used by numerous companies in the past but you *need* a source license and a lot of work in order to get anything up and running for a massive online game. Also I really don't see any benefit to using UE3 over something tailored directly to the genre.
Star Wars TOR (heavily instanced / story based), The Repopulation (complete open world sandbox), Faxion Online (traditional type MMO) have all been built on HeroEngine and as I mentioned in brackets they are extremely different types of games. I'm not sure what you mean by "too managed"? The tools are extremely in-depth and give you everything to build, run, and manage an online game. Also note that Zenimax Online licensed HeroEngine for The Elder Scrolls Online just for the use of the toolset so they could get started on world building, design iterations, etc while they build their own custom engine.
(HeroEngine has a great feature set, but seems to be too managed for an MMORPG)
Another great choice is to use a combination of Unity and a socket server like Photon (definitely the best to pair with Unity). You can download the free version of Unity and Photon and get started right away... plus they even give you a small sample to get you started quickly before you have to dig into the docs. Unity has been used for MMO games such as Battlestar Gallactica, FusionFall, etc... so I don't see why there is an issue there either.
Actually HeroEngine and Big World have a massive number of AAA MMO games built using their technology and the above you listed really only have a couple (being a few that use a modified UE3... the only MMO I count for using GameBryo is Dark Age of Camelot as the others use a version that isn't even recognizable. Meanwhile UE3 hasn't powered that many either and the ones that did used it had to heavily modify and add to it as well.
The reason I've limited my list to three is because no other engine on the market has been proven to work with an AAA MMOG.