It occurs to me that a desirable support product would be some kind of standard character storage record format -- and software -- that would allow one to down-port and up-port characters from a modified Reverse Engineered server (whether that server was actually emulating City of Heroes or some completely different game developed to use the I24 COH client software).
One thing to note... in one UO shard where I coded, I wrote a magic system. This system involved memorizing spells from your spellbook rather than casting them from your spellbook. So your character would have a list of "spells in memory" as a characteristic (or as a statistic, whichever you prefer). When the character is saved, this list of spells would have to be saved with them. Now, this is a custom magic system... another shard would not have it, and therefore when the character data is presented to them, they have a large section of the data that is incomprehensible. With most coding styles, this would crash the load, and prevent the character from being loaded.
There are some work-arounds that would mitigate the situation (for example, you have to put the data's name, and the loaders for all of the custom servers look for a name that would tell it what to do with the upcoming section of data). You would still have the issue where a mage from another shard would not be able to cast any spells in this shard since he didn't have any spells memorized, but at least the statistics that are common across all of the shards (or servers) would still function. And it's assumed that if you actually got agreement about how to write a "accept anything" character saver/loader, you would also agree on standard values so that you don't have one server using unusually high values of HPs or Endurance, and messing things up when characters from that server are loaded elsewhere.
Basically, the point is this: the more variety and customizing you allow to each server, the less interoperability there will be, even to the point where (like the private UO shards) characters are not transferable in the slightest. But requiring interoperability might stifle creativity and custom ATs, powers, etc.