Formal:
* Meanwhile
* During this time
* In the interim
* Until then
* Pending further developments
Informal:
* For now
* Until then
* While we wait
* In the meantime (this is still informal, but less so than some of the others)
* Until further notice
More specific:
* While you're waiting (if the context is about waiting)
* Until further instructions (if you're giving instructions)
* While that's happening (if you're talking about something else happening simultaneously)
Ultimately, the best term to use will depend on the specific situation.