Brian,
Here are some thoughts about new rooms:

Flooring
I would want all the rooms to be carpeted except the art room and the kitchen, if you have one. This makes sitting on the floor sooo much nicer.

Plan Ahead Decorating of Each Room - Paint, etc.
Select the theme for the room and paint the background color appropriately. For instance, we have a Temple School room painted gray with darker shades of gray and black used to make it look like a stone block room. Planning ahead will save you a step in the process. Each of our rooms needed base coats of one color or another before we could even begin painting murals.

Possible Decorating Themes
We chose biblical themes in almost all our rooms so that we could use them in multiple ways.

For instance, during Lent we did an intergenerational "Journey to Jerusalem" using our kitchen (Martha's Table) for the Last Supper.

The Temple School for Jesus driving the money changers out of the temple, etc.

It was a good way to get those people who never step into Sunday School rooms up into them.

Sinks
Sinks in the art room, as Brenda suggested, are a must

Storage & Cupboard Space
Storage in all the rooms is a definite plus, especially when you have to put away everything in the rooms for some event (like a rummage sale). Cindy