I've been working on a mod for gits and shiggles, and an looking to make a custom campaign around it. I've been thinking through the tech tree and wondering what you guys think or what's missing.
Stalker (Co-op one)
Dark Templar (co-op, but no void stasis)
Dark Archon (Brood War inspired has mind control, feedback maelstrom, made from DT)
Disruptor (Co-op version)
Oracle (modified LotV version, permanent revelation, pulsar beam is standard attack)
Scout (range upgrade, stun missile upgrade)
Void Ray (Requires Fleet Bacon)
I'm including the scout since the dark templar have their own versions of them, and moved the void ray to fleet beacon since there is a lot at the starport. I'm not sure what I could do to make it capital ship worthy though. The Oracle has permanent revelation for a mission idea I'm thinking of where you'll want to keep track of dropships attacking your base. The disruptor is meant to serve the role of Nova's drone for co-op, sneakily taking out buildings like missile turrets.
I'm looking putting emphasis on smart use of units like in Brood War over the mass all the things in SC2. (Yes I know that was a thing in BW too but it wasn't ideal due to controls).
Well, the basic formula for making a regular ship into a capital ship is pretty simple; raise the price a bit, beef up the shields and health, and in the case of a void ray, I'd go for either a slightly longer range or increased damage. The feature that makes void rays special is, if I recall correctly, that the longer they shoot, the more damage they deal. Doing something to amplify that effect in some way would likely help make the void ray a more powerful unit, on par with a carrier or tempest.
If you're up for a bit of data editing, making the void ray model slightly bigger might help as well.
Another possibility would be to invent a new Dark Templar capital ship; I'd base it on the void seeker model (you may need to scale it down a bit), although I don't know what to give it for an attack and/or ability (or just give an additional special ability to the void ray.. Perhaps a medium-longish range blink ability? Capital ships usually hang back and attack from a safe distance, either with launched interceptors or the long range tempest attack; being able to jump right on top of your target, and to then quickly blink away to a safe distance (ideal for escaping static defenses) would provide similar tactical advantages to a tempest that can attack directly from a safe distance in the first place.