I love textmate to death and use it exclusively for all my development since its release.
The only area where I wish for some improvement is the file management. Most of my work is in projects which contain hundreds of files and folders so this limits the tab bars usefulness quite a bit. It seems to be easier for me to start several textmate instances one pointing to different logical parts of the application.
The first and bigger proposal is to bring intelligent file opening to textmate. This works much like quicksilver. A hotkey opens a edit window and an attached dropdown window specializes itself more and more as you type. A simple search would be "user" which would offer you all files with the word user in them. A more complex example would be typing "usrg" with would find USeR_Gateway.rb, USeRGlobals so on. I recommending installing quicksilver for a demonstration of the paradigm. Its great emerging technology and would be a great competitive advantage to have natively in TM. The same code could later be used to jumping to methods within a file.
Another proposal would be introduction of intelligent tab fading. During inactivity the least recently used tabs start to fade out and close themselves after a while. This happens slower when there is lots of space in the tab bar and faster the more items there are. This would be great to cut the tab clutter and get rid of files automatically with which you are done. Currently i'm finding myself using close all windows all the time.