I really don't think it's necessary to have a Cap slot used. It's going to affect capital ships and it will displace the purpose of starbases.
Capital ships are already in an unbalanced state. You get more bang for your buck to use frigates > capital ship (in most cases). Having a Starbase use up cap ship slots means we're only going to see fewer capital ships and fewer starbases. I really don't want a game with 8 captial ships and 8 Starbases. I think many Sins players will agree with that.
We want a lot of capital ships blowing the shite out of everything!
Furthermore, starbases are meant to be used as fleet support. They are not a mainstay, or the backbone of an offensive or a defense, fleets aka ships are used for that role. A starbase is meant to assist those ships by providing more fire power, producing ships on the front lines, boosting the economy and other non-engagement roles.
We gotta keep in mind why Entrenchment is being brought into design by looking at the problems we have now and the slutions being brought forward.
Problem 1: Original defenses were not very useful even at the most upgraded stages. Medium size fleets could easily overwhelm them. We needed something to boost these. This is being done by adding flak to hangars for instance or increasing the range of turrets (damage to beam turrets etc).
Problem 2: Online play showed that it was very easy to bypass all worlds and unreasonably pillage homeworlds to the point of frustration. There was nothing to stop opponents from running to the innerworlds and destroying them while our offensive fleets were on the other end of the system. Starbases are meant to solve this problem. So we need STATIONARY FREAKIN' STARBASE** (LOL) to either stop fleets from bypassing outersystems, or, one that sufficiently guards our inner worlds with the backup of the new defense structures.
Moving them into a fleet type role is going to offset the balance of the game in a negative way. Lets not leave starbases in an undefined category only to have them never be used. If we want super capital ships with heavy fire power, then ask for that and demonstrate why they are needed, but what we've asked for is a defensive structure to help defend our inner worlds or stop fleets from bypassing our outer worlds.
*Side Note: Problem 3 was the AI. Tactical, Strategic and Unit AI. Which I think they've improved on immensley and I hope they have a few more surprises.
** For anyone who doesn't know: https://forums.sinsofasolarempire.com/333087/page/7/#replies