I feel that for modding purposes SEV is very much lacking these 'build requirements' that many other strategy games have. I think this feature would greatly increase modders' freedom( by limiting the player's, ironically ), and would like to see how popular this idea is.

Examples:

Let's say you want to build a Heavy Cruiser. With the current situation, all you need to do is to research the technology, design the ship, then put it on any colony's build queue.

With build requirements, modders could, for instance, limit the construction of Heavy Cruisers to planets that have built a 'Orbital Ship Construction Facility'( Name's obviously just an example ). An organic Heavy Cruiser might additionally require a 'Organic Factory' or some such.

This could be made to work with weapons, engines, etc. too, and it could even be connected to planet specials.
Let's say you invented a new great weapon, which unfortunately needed specific materials to be created. With this system in place you could limit the construction of ships/units that contained this weapon to colonies which had this material as a planet special.

This could also, of course, be used for buildings: To build facility X you need to already have built facility Y, etc.

These are just a few examples of how such a feature could be used. I think it would add more interesting gameplay and help to create some more diversity between colonies.