Manufacturing and Research Slots
With maximum skills you can only have 11 Research and 11 Manufacturing slots and these can disappear very quickly especially if you move into advanced manufacturing/research (e.g. Capital ships or T3 systems/ships).
The limit is annoying even for T1 items, they consume research/production little time compared to other items that you're usually able to do more with them in a day. Problem is we can't be at the keyboard 23/7 and so slots are lost due to pending jobs or, in public stations, slots are lost to others.
Adding a couple of new skills, much like trade skills, that adds extra slots would be the best way of adding this. Making the skills time intensive and not something that can be trained in a few hours is how I'd implement it.
Industrial Management (6x) - adds additional 2 manufacturing slots per level
Planetary Industrial Management (8x) - adds additional 5 manufacturing slots per level
Research Professor (6x) - adds additional 2 research slots per level
Einstein Fellow (8x) - adds additional 5 research slots per level
Everyone comes out of this better, industry types get more slots to do work in, production (potentially) increases as more can be made/researched and the market gains more product.