I agree with SK, Geothermal needs some love. It's nice but it isn't even "that" great for providing energy (which it should if that's mostly all it's going to do). Without counting techs, it provides a bonus of 5 energy. If that tile was bare, an early generator could've provided 2 of that. So, it's nice early on, being able to work 2.5 generators with only one citizen -- but it's not great. I won't even count other things, like floatstone quarries which provide 3 energy, because those come later on.
I think the geothermal yields should remain as they are and that some new uses be applied to them other than being a requirement for a few wonders.
1) Turn Geothermal into BE's version of Coal, and quantities are consumed of it to construct some awesome production buildings in cities (non-wonder) (BE's version of Factories).
or
2) Turn Geothermal into BE's version of Marble, and have it grant a production boost when constructing wonders if it's within a city's workable area and owned by it.
or
3) Tweak existing quantities of Geothermal on the map and have all wonders require different quantities of it for construction. Geothermal becomes sort of a "wonder currency" to spend on them. This will help fight against wonder spamming and players have to pick and choose what they want to spend it on or else acquire more Geothermal. Wonder balance can be more flexible now since geothermal costs can be decreased or increased to match their powers. Also, this would make Geothermal into a nice thing to trade for.
(Though, I know a few wonders aren't exactly constructions -- not a big deal, in my opinion.)
I think #3 would be really interesting if it was made to work. Though, I'd probably be happy with #1 as well.
EDIT:
4) Really powerful tile improvements (maybe even affinity-specific?), somewhat like great person ones in Civ 5, whose maintenance costs 1 geothermal (or others could even be more if they're really strong, but they would be limited for each faction). I think this could be a really cool thing as well and make the map even more interesting to look at. Plus, it adds a little more depth to the terrain improvement part of the game.
So just like you spend other strategic resources on strong, mobile units to fill the map; you spend geothermal to power these strong, immobile improvements within your empire.