Developments that have a negative effect, or which you still own even after the planet has been conquered :
You know that your planet will be invaded, so why not using one of its slots to create neagtive effect, or install a spying station, smugglers which will divert some of the ressources to you, eat stars, or just use a slot … But of course, if the ennemy does not take the system, you are the one which will suffer the effect.
Going further, we could have “drop ships”, which could drop such (or any ?) development on contested planet.
Finally, we could have cards allowing us to bombard our own planet, to get rid of such nasty developments, or to destroy our juicy shipyard, just before our system is taken over.