I would like to see the ability to build lower grade structures than what's the highest available. Sometimes you still may come across a planet in mid-game which hasn't been touched yet, or you may be lucky to get some heavy tech bonuses and race through some of the early techs. When you colonise these planets, you're forced to build the highest level of what is available, and I hate that. You're either forced to pay a lot of money to purchase it right away (unless you do the payment plan) or you're forced to wait an ungodly amount of time for construction to finish.
You should have the option to build any level of what's been researched so far, so you can at least quickly get some basic structures down without having to wait forever.
I hope this hasn't been suggested before, but I haven't seen a thread on it yet.