Dumb I don't think a turreted ion beam would actually be over powered because ion beams are so expensive.
In terms of damage output per credit they are actually terrible in comparison to cannons, not just because the ion beam itself is expensive but also because it pretty much needs a dedicated reactor. The current strength of ion beams is that they can be put deep inside a ship where they are incredibly hard to destroy.
To continue with my original idea being able to build on turrets would make creating customized turrets possible.
The actual guns could be separate parts from the turret itself make it so that instead of needing to create new parts for single gun, dual gun, quad gun etc, one could put together different numbers of guns on generic turret mounts.
This would have the added benefit that for big turrets they could be partially destroyed by having some of their guns knocked out. In contrast to not being effected by damage until the entire turret blows up, as is currently the case.