[turbo6bar]

There is a lot of great advice here. Here are mine, which do overlap with other comments:

1) Focus on quality space, as opposed to square footage.
2) Conduit in selected walls, so you can upgrade network, audio, communications as technology evolves.
2) Don't overlook insulation. Thick walls cost very little to implement, but energy savings can be immense. If your HVAC system will be in the attic, consider an unvented attic space.
3) As others have said, wider pathways and doorways. You may need the girth for a wheelchair at some point in the future.
4) Consider serviceability in the future. That second story looks great, but gutters are a pain to maintain, and painting becomes a real chore.
5) Hot water recirculation pump is easy to add during construction. You can trigger the pump on a 30 second to 2 minute timer via low voltage wiring to the bathrooms and kitchen.
6) Oversized garage to easily hold a full size vehicle plus toys and tools. Supplement with a real shop separate to the home.

To stay on budget, stick with your plan and don't underestimate the cost to finish the home. If the concrete guys do a lousy job on the slab, the tile guy will want more bucks to get your floors flat. Don't let poor management in the front stages lead to cost overruns later.