I think that systems should be able to produce more than just one building or unit at the same time. It would add more flexiblity in what goals you can be working towards at any given time, and it makes sense that an entire solar system spanning multiple planets would be able to divide its attention between two or more projects. It would not be easy to get another production slot, and it would only be available in your more developed systems.  This would also make the  to  continuous conversion projects (I forget what they are actually called) more appealing to use, because you don't have to completely stop all other production on a system to be using them. Of course, this might not work because it would probably lead to a crazy explosion in fleet sizes. However, if using two slots brought each slots production efficiency to 60-80%, with further active slots pushing the efficiency lower, then that might curb that problem.