Adding a heat system allows for smarter designing of builds, and makes death cubes obsolete, which I'm sure everyone except the annoying people who build them happy.
Death cubes are OP because of turning rates depending on the dimension size of the ship + they have the lowest ratio of surface area to their volume, which is nice as long as the hull blocks are useless and doesn't make the ship any stronger.
Also the distance from any point from their surface to the core is pretty similarly big.
There would be changes, to the thrust/turning and HP system, most likely making cubes as strong as any other ships.
Of course cube ships would be still very easy to build, but if anyone wish to fly around without a style, then it's theirs choice.
Heating system (by Arkudo idea) could be easily abused by building blocks sticking outside of the boundaries of the ship. 100 blocks long stick and i can jump wherever i want.
I think much better idea would be to add blocks which would be responsible for cooling down, maybe their efficiency could be dependent on their number and the surface which they are covering.
But i'd still keep to the idea of not using heat.
MrFURB I think the FTL would be easily adjusted via block config. Players would just need an option to select that once some requirements are met, the FTL can be activated. This requirements could be i.e. some energy stored in FTL blocks for every block of the ship or some cool-down timer after/before using, range of jump, etc. And of course combined requirements.
Also if we could implement another types of resources (heat, fuel, ammo, whatever we wish to call them) and that certain actions would either deplete the store of specific resource or add the number to it, then we would have pleeeentyyyy of various combinations, for thrusters, power gen, weapons, FTL, etc.