Is there a reason to make non-cubic designs a good design (if not one of the better choices) that isn't an argumentum ad populum (an argument based upon using popular opinion as the basis)?
A heating mechanic would certainly be interesting
But at any rate, that'd be better served as a Suggestion thread.
Again, is there anything wrong with people building cubes? If, as a thought experiment, you were to make it so that people got the best performance out of building ships that looked like what you envision as an ideal space ship, would you have the same problem with that as you appear to have with people making cubes? Because it's the exact same problem (there is a simple ideal design) with the exact same results (people make their ships to fit this ideal design).
1) The game is designed to sell copies of the game, to provide the author with a profit and an adoring fanbase. Thus argumentum ad popularum is perfectly valid because a more popular game sells more copies.
2) You're overthinking this. Humans like to build gardens, absent other constraints. When we build something other than a garden it's (usually) because other factors are more important, such as the wheels needing to touch the road and cost of materials. You are brainstorming methods by which non-block shapes could be mandated or incentivized, which is wholly unlessCary unless your objective is to punish people who aren't creative/are utilitarian.
You don't have to overthink this. If you make shape not matter then people will build and use awasome looking ships. There will be some optimal 'ratio' or set of optimal solution 'ratio's but any ship that uses those ratios will be just as effective in any size. So if you want to make a block do it, if you want to make a spirally thing with a happy face you can do that too.
So make shape not matter. Revert the integrity field change.