Nice topic indeed.
In addition to input above my short input:
1. Consider travel
Establishing a branch to cut costs? Consider travel expenses. They will be unavoidable.
2. Time zones
Choosing a location (in case it is possible) consider time zones. There are 2 sides to this:
a. Having a location close to HQ time zone is critical for collaboration. Otherwise be prepared for all processes to have at least 16-24 hr. loop.
b. On the flip side, you will be able to reach out/provide resources to users/clients/audience located outside of your current geo reach.
3. Team communication
In one of my startups I've noticed a communication problem rather late: our HQ was located in a multi storey office complex, and office space was distributed across 3 floors. At some point I noticed, that team members from one floor don't recognize the ones from the other floor: they simply did not have a need for a direct communication (engineers and accounts or HR and design).
Needless to say that international offices where known only from office pictures on "contact" section of the company website.
1. Enforce visual communication (in my case it was cisco video phones for security, but skype video is quite enough normally);
2. Weekly breakfast (every monday regional team had a joint breakfast in a nearby coffee joint);
3. Global calls at least 1/week. In our case it was a team call every Friday on result achieved during past week.
4. Management visibility
Generally, it is critical for ANY company to understand where the management is. Especially in Startups.
To an extreme in my case: all my employees had a visibility of my current location on "Find Friends" app. Which might have been an overkill.
Absence from office(s) is not a good sign (I know this first hand as the one being absent during complicated periods and knowing the negativity of this).
5. Mission clarity
It is extremely critical that teams spread across locations are on the same page in terms of company status, current focus and priorities.
Don't be shy to allocate as much time for this as it's reasonably necessary. Overshare will not be your problem unless you try to isolate divisions.