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.
Specifically, I want to make a terran based faction of custom units; with the premise that instead of building your army on site, your army is pre-built and deployed onto the battlefield. So, given a supply cap of 64 supply worth of ground units (not counting SCV's) and 32 supply worth of air units (none of which can be battlecruisers), what would you want?
The concept I'm trying to implement is that you have one or two battlecruisers that you deploy units from, rather than building them at a factory or something. (and deploying a battlecruiser from a battlecruiser is just absurd) You can only replace units, you cannot build more of any unit than what you started with initially. So if you only have 4 siege tanks, you can only ever have a maximum of four siege tanks at any time.
So what would you want?
Should this become a campaign, you could expand your forces after every mission, and get extra forces as a reward for successfully completing bonus objectives. I'll need help with the plot, unit balance, and map making; I just thought this might be a fun concept to try and implement. I'm finding that making custom units and abilities is a lot more fun to me than building and terraining maps; suggestions, and ideas are welcome.
No promises on the timeline, I only started playing with the editor quite recently. I'll post this as an actual project once I've got something that sort of resembles a functional map.