I just thought of an idea to give more use out of low class planets- a Mining Improvement. Instead of adding to production and research on the planet the improvement is on, it boosts production and research on another planet in the same sector.

When you first build one, it will generate a special ship called a Mining Convoy that you send to the planet you want to be boosted. To balance it, it a mine would have pretty high maintenance costs, but would give a percentage bonus to the production and research of the destination planet. Also, to simulate the harmful effects of mining, for each mine you build on a planet, you get a penalty to the effectiveness of each non-mine improvement on a colony. For example, if you have a mine on a planet, then you build a factory, the factory is less effective, than it would be if there were no mines on the planet.