Merging brains, copying thoughts and types . . . this will be great!
Inspired by Trischa's predicament, I would like to suggest an ability to include sub-brains in my main brain. A sub-brain is a bonafide brain file in its own right, but when included in another brain, its thoughts are accessible as read-only. I wouldn't be able edit the included thoughts at all -- thought name, notes, attachments, etc. -- and I wouldn't be able to drag links to or from them. Searches would, however, find thoughts in the sub-brain.
In this manner, what appears as a single large brain is actually partitioned into several smaller, independent and specialized brains. An application of this kind of structure might be a brain of the USA in which I create a Colorado brain, you an Oregon brain, and someone else a tiny little Rhode Island brain. The larger USA brain would simply include the state brains which could be separately updated and enhanced, and which could remain the IP of their individual authors.
It would be useful if the sub-brains could also include sub-sub-brains which would also be accessible in the main brain. To continue the example, a California state brain could include San Francisco and Los Angles sub-brains.
Actually, most of this idea of included thoughts is stolen directly from the TiddlyWiki (TW) IncludePlugin which I used to divide a large TW into a number of relatively static parts ncluded at load time into my main and quite dynamic main TW.