All functional modules (engines, capsules, etc.) will produce heat as they are used. If too much heat is produced by modules or from re-entry can not only destroy parts as it does now, but also cause parts to function poorly or stop working entirely. Heat can be removed from parts using specialized parts.
heat sinks (stores a set amount of heat, then is ejected, one-time use, works anywhere, removes a lot of heat quickly, very light.),
fans (only works in atmosphere, heavy, gets rid of a lot of heat, works for as long as you have electricity)
radiator (works infinitely, gets rid of a small amount of heat, works everywhere but decreased efficiency in atmosphere.)
Also suggestions tag isn’t actually a development tag but is a tag to suggest mods. You have to put a suggestion in the development section.