It would be wonderful to have a range of upgraded turrets where the main upgrade would be the ammount of bullets stored.
There are several large ship mounted weapon systems which carry 1k+ rounds (belt fed or rail fed). Having something similar for our bases would be wonderful.

It would also be really nice if we could reload turrets via conveyors. Rimefeller has an oil delivery system using pipes, if we can mod a similar system to carry bullets you could create a nice bullet factory, feed the system through containers and automatically reload the connected turrets through that rail system.

Since this would allow for some extreme level of protection i guess it would have to be relegated to late stage and most likely cost a pretty penny in research and production to make it not instantly gamebreaking.