+2 for a the Base Defense upgrade.
But I'll take it further. for a long time ago i wanted skirmish mode named "Research" , where you only can build extractors, factory(all units are locked at start), and a quantum relay for collecting quanta to unlock new units and buildings aka: Supcom 2 and/or The Moon Project(pretty old game, but i loved the research there). There will be a tech tree. In this mode advancing are considerably slower than normal skirmis, but sometimes i want to research , upgrade, not just rushing.