Just a few ideas...
IM - SMS -> send offline IMs to a cell phone as a text message. You could probably already script up something to work with your email client to do this...
Offline sandbox -> Be able to build in a local offline sim, then upload the results to the grid later. Good for grid offline time, or if your own Internet connection is down. People that depend on SL for financial reasons would benefit from this, since downtime wouldn't prevent them from working.
LSL compiler and interpreter/simulator -> I'd *LOVE* to be able to script from my laptop, which can't handle the SL client. You could have the simulator create "objects", and view the effects of the script on the objects.
These ideas wouldn't even require an open codebase, just open specifications (the networking API and the scripting language's underlying implementation)... Heck, we could probably build the LSL tools without knowing anything more than we know now. Might not work *exactly* right, but it'd be useful for prototyping.
Thoughts?