The garage will be primarily for vehicles and activities related to them. It could be used for other outdoor work/hobbies, but we currently don't have any.
The main house will have a single primary entrance for both residents and guests. The garage, however, needs two distinct entrances: one for people and one for cars. A garage door does not work for people — it is the wrong scale and feels awkward. That entrance operates mechanically, via a remote. This works from a vehicle, but feels slow and annoying to a pedestrian. To accommodate people, we will have a second, standard sized door. Ideally this will be connected to some kind of covered walkway that leads to the front door of the house. For most single family homes, where the garage is right next to the house, this would be a short gap of less than 10 feet; but in the Taltree development it will likely be longer.
The garage should have space for parking two cars. There should be enough space on all sides of the car to comfortably load/unload people and parcels. (Including the front — while most cars load people from the sides and parcels from the rear, some cars, such as Porsches, have storage space up front and the engine in the rear. Even more interesting, the Tesla Model S has all the hardware in the floor, so both front and rear are storage; and the rear includes two seats for children.) The garage floor should be level and smooth.
The interior should have good lighting. Light sources should include a mix of natural and artificial sources. Most garages are fairly dim; light makes them feel more comfortable and welcoming. It also improves safety and makes it easier to load and unload the car. We would like to have the lights on a timer and/or sensors, so they automatically come on when entering the garage and turn off once everyone has left.
We like to clean and detail our car(s), and being able to do this in the garage would be delightful. It would allow us to keep the car looking good even in the winter or when it is raining. Detailing is about more than making a a car look nice — wax or paint sealant helps to protect the metal and paint from water and dirt, so regular detailing can help maintain the car. For detailing, we need a drain and hose spigot inside the garage, plus ample surrounding space. An electric outlet would provide power for detailing tools.
I like to work on my cars. In addition to work space, this requires storage space for tools, jack, stands, parts, etc. Both overhead and movable task lighting are required. Both lighting and tools need regularly-spaced outlets. We will also need storage storage space for wheels/tires and other seasonal items. Car work can be dirty, so an in garage sink would be nice.
We will need space for cabinets, drawers, and other organizational aids to store supplies and tools. We may not install all of the cabinets we want at once, but it is important to design in the room for these things.
Looking forward, we want to be able to eventually install an electric car charging station. A normal household circuit is 120V at 15 or 20 amps. Large appliances (such as washer/dryer) might run at 240V/30 amps. Electric cars can use a larger circuit to charge more quickly. For example, Tesla's site lists home charging up to 240V/90 amps; and it has support for 440V commercial circuits (I don't even know if that is possible to get installed in a residential home). Getting a very high-amp 240V circuit run to the garage seems like a good thing to have done up front.
Even if it is not part of the main house and is a more focused space, the garage will have a big impact on the quality and enjoyment of our living situation; so making sure to think it through carefully before we start building is important.