I love the idea of geothermal. There's a zero % chance of carbon monoxide poisoning, but the price in my area was insane. Even with all of the state and federal credits. I honestly think that everyone raised their prices higher, to soak up the credits and as soon as the credits end, the prices will drop to how it was with the credits.
My house is a bit more difficult as an HVAC problem. I have to heat and cool 1662 sqft. But then I later have to heat and cool 4162 sqft. This results in two installations. One for the existing and another for the new portion.
Add in the future 2500 square feet in the basement and that's a whole other complication. But i don't necessarily want to heat and cool the whole basement. I want to do just a few rooms and only on demand. My office, the gym, my wife's room and then the home theater. Home theaters are a whole separate animal. They have no heat load in them, or a very small load, but then add ten people and you have a huge load. I've been talking to my installer about using a Mini split for this situation. I'll be able to heat and cool these 4 rooms on demand instead of always having them conditioned. Considering the basement is 8' below grade will bring about some natural climate control from the walls. But I won't know exactly how that is until the whole house is sealed up.
But this will all be a fun problem to have after working outside for the last 4 months.