You need some space for working on instrument repairs. Counter space, and a giant sink with hot water for cleaning out brass.
Make all of your storage rooms as big as you can. Plan for growth.
How will you get from the band hall to the marching practice field? Football stadium? Auditorium? Anywhere else you rehearse or perform? Make sure your pathway is clear. No stairs. Wide doorways. If there's a center bar in a double door, make sure you can remove it with a key (no tools). Standard-width doors will not accommodate your largest timpani or marimbas.
I'll second the plugs in the floor. Plan out where your podium will be. Put a plug in the floor very near it for your Harmony Director or whatever else you use.
Plan a sound system that you can tie into. Ideally with an input right next to the power plug next to your podium. You want to put the metronome behind the band, but control it from your podium.