There's one important thing I forgot to stress: the test version has 10 BFRed-equivalents + a superheavy cannon.
!!The final version will not have such firepower!!
I cannot tell you exact numbers, as I haven't tested this yet, but the combined firepower of the front cannons will be enough to take down a Sathanas in a reasonable amount of time (somewhere between 5 and 10 minutes). The key is not pure firepower, but lower refire rate, which was a problem with first generation beams.
Shields: It seems illogical to me that the Shivans did not equip their most powerful ship (at least what we know of), the Sathanas with a shielding similar to the Lucifer's. It's also quite strange that GTVA adaptation of shield technology halted at fighters. Now that FS2_open supports capship shields, why not implement them?
The Shivans have at least 80 juggs which can pulverize destroyers in a matter of seconds. The GTVA doesn't even have 80 destroyers! If we don't want the FS story to end with the annihilation of the Terrans and Vasudans (and I, personally, don't), we have the following options:
a) build 80 Collossus-size superdestroyers. They'll be ready in a few centuries
b)build 800 destroyers and swarm Shivan juggs 10:1 (possibly from the rear), same as above
c)develop new armor which is 10-20 times more powerful than current armor, including the Shivans' (which is not better than GTVA armor). How much did armor technology advance in the past 30 years? Almost nothing. Okay, we've got CC Molybdenum, which is thinner but just as strong as the older armor types, and the Hatshepsut is 25% more powerful than any other destroyer, but that's it.
d)develop capship shields. I don't know what the average shield power/hitpoints ratio is in case of fighters, but for destroyers, it would have to be much higher, shields would have to be 5 times the Colossus's hitpoints in power. Otherwise shields are down after the second or third shot and the ship is toast after the next. Because the FS2 ai doesn't use the shield equalizing function too often, I mutiplied this by 4 in case of the aurora (it was 6.25*4 =25 million). Once again: this is still just the test version!
The base problem is that beams in FS2, especially Shivan ones, are too powerful compared to the hull of the ships, which reduces combat with shivan capships to something like this:
"GTD destroyer>Command, a Ravana class destroyers has jumped in. They're firing at us
Command>Engage the destroyer, we'll send in reinforcements as soon as possible
GTD destroyer>Hull breach on multiple decks, I don't know how much longer we can...
Command>Oh well, send in Alpha 1"
This is ok for FS2, in which the GTVA gets their butts kicked by the Shivans, but in the future, the GTVA should improve and make ships which can compete with Shivan designs.
Another reason for using shields is that they can recharge in a few hours. To fully repair the hull of a destroyer would probably take weeks. Also, shields distribute damage around their entire surface and protect the entire crew even if they're about to fail. Armor doesn't, and a well aimed shot should kill a few hundred people even if it's the first shot (windows, e.g., cannot possibly resist that firepower).
Power source: experts predict we'll be able to build working fusion reactors in about 50 years. Nuclear technology will be more than 100 years old by then. In 2367, GTVA ships still use fusion reactors, but the alliance already has anti-matter bombs. Therefore, I think that a suitable successor to the 300 year old fusion technology would be anti-matter. The Aurora is an experimental heavy beam destroyer. It's fighter capacity is minimal, it has a crew of only 1000-2000 (why do we need 10000?). It's equipped with a matter-antimatter reactor and an auxiliary fusion reactor. The fusion reactor is enough to power life support, engines and the warp drive, but shields and weaponry won't be of much use without the main reactor.
This is just my idea of the future. But if you want to stick with FS2 technology, then I'd say a hull of about 150000 (to show some minimal advancement), 10 SgreeenIIs with a lower recharge rate (15 sec or so), and an LRBGreen as the main cannon (maybe with a slightly increased recharge rate). The remaining cannons all around the ship should be similar to vasudan slashers. It'll beat a Ravana. If a Sath comes by, we'll send in Alpha 1 in a Boanerges.