Taking the real-life example of WWII torpedoes, for example. Ship could always launch torpedoes...Once aircraft demonstrated the ability to drive home these devices with a degree of accuracy not possible from ships, the results were devastating. Taranto, Pearl Harbour, Midway. Bismarck, Yamato, Arizona. The most powerful ships and the most powerful fleets were destroyed by little tin cans.
Aircraft launching torpedoes against warships =/= Starships launching torpedoes against other Starships. Sea combat has transitional craft - submarines under the water, warships on the surface of it, and aircraft above it. Aircraft take advantage of not having to deal with ocean-drag and are incredibly fast, but small and light because they must expend fuel to stay airborne. Submarines take advantage of being very hard to see or hit, but also have a hard time seeing and hitting things and are slower. Vessels on the surface don't combine those two, they do something else and have heavier armor and armament, meant to deal with all threats as there is little to keep them from doing so - floating is free and more mass doesn't really translate to slower ships.
A better comparison of fighter craft to space combat is that of a PT boat or motor boat engaging a super-battleship in the open ocean. Good luck with that, especially when there is nothing stopping that battleship launching PT boats/motor boats back at you if they're so effective.
If very powerful but slow dumb-fire-only weapons are made available, fighters could be very viable. Especially when you consider that not dozens, but hundreds of fighters or drones could be produced for the cost of one moderately-sized capital ship. PD isn't necessarily an instagib weapon either, it depends entirely on how the mechanics shake out.
If those hundreds of fighters are the same block-count as my one cruiser, and the blocks are all additive, I have hundreds of times the shield capacity as any one fighter. Now, we've got the same DPS, possibly relative acceleration, and armor, but my shots are more likely to punch through - any one of my volleys is hundreds of times more than any one fighter's volley. If I can't insta-gib any one fighter, hundreds of fighters would need to fire hundreds of times each to hope to damage me.
Now, it's also reasonable to assume that I can not only mount equivalent weapons, but also larger and more powerful ones, in addition to having a larger, more powerful reactor, giving me more
DPS than any equivalent number of fighters.
Another advantage of fighters, (sort of touched on by the trench run example) is that because of their maneuverability, they may have the freedom to attack a ship from any angle, allowing them to hit your weakest points. Whether that's the rear of your ship which is mostly covered by engines, a blind spot for most of your point defence (or a place where most of them have been knocked out), or a place where your superstructure is exposed that you rolled away from the enemy capital ships to protect from fire.
I doubt you could do much to allow fighters to 'sneak up' and hit a weak spot if you're using anything like a space environment. I can roll faster than you can cover 10,000 kilometers; shoot, I can probably roll faster than you can cover 100 kilometers.
Fixing weapons' engagement ranges to some distance that fighter weapons can hit and making them slow might help the fighters' case. I'm a big target and dodging all those shots is going to be hard. The fighters could spread their attack out over a sphere, forcing me to fire in all directions and not concentrate in any one area, but then I can always rush one side of the sphere and punch through since their firepower isn't concentrated either and the fighters on one end of the sphere would not be able to catch me.
The result is that I escape having done more blocks of damage than any similar number of fighters.
I'd expect lasers (if we get them) to be the least efficient weapon available by a considerable margin. If they weren't, who would use anything other than lasers, since they're a hitscan weapon, giving them effectively perfect accuracy?
Even if they're inefficient, if they're anything like reality range-wise, they'd be killer. I'd consider using physical-based weapons if lasers were too inefficient, but engagement range for those is still higher than fighters could be comfortable at - consider the OP and beyond-horizon weapons we have today
Nothing is guaranteed, whatever makes for better gameplay will be used. I'm not saying it should be as clearcut in favor of aircraft as late-WWII was (I love battleships as much as the next guy (unless the next guy is Snowdragon)), but there's no reason fighters shouldn't be made viable.
I'm for better gameplay; but I don't see how you can make fighters viable without doing something to make larger ships unviable. Fixing power generation per player, creating limited hyperspace/FTL capabilities for ship-systems, reducing efficiency as you get larger, etc. And I don't know that fighters have
to be viable to make for good gameplay.
One thing that did occur to me is to create different 'spaces' and allow different sized ships (or ships expending different amounts of energy) to enter them. You'd maybe be turning sci-fantasy, but what if you had the following:
Real-space: What we know and love, favors super-large ships over anything else by nature (so far as we've discussed, anyway).
Sub-space: Has an energy cost associated with entry, but not necessarily with staying there as long as a ship has a blanced sub-space signature (perhaps lack of shielding, or a certain size-to-energy ratio). Ships within are difficult to detect, but also see the world as shadowy. Travel is much slower than in real-space, so all weapons are much less effective (slower, less damage dealt). Explosives, however, are much more effective against ships in sub-space. Explosions are normally about the least-hazardous kind of weapon you could have, for a given energy output, since the whole force is distributed around the sphere of the blast. Here, explosions are many times more effective.
Super-space (Hyper-space): Ships within are much faster - FTL kinds of speeds. Most ships can only engage the drive for the briefest of seconds, to conserve the massive amounts of energy it takes to use. Small ships, however, can enter and stay within (with certain limits), provided they burn through fuel at an astonishing rate (you burn exponentially more fuel if you have more mass). The limits are that ships remaining within are visible to real-space ships, objects released will revert to real-space equivalent velocities, and actually being hit results in instant collapse of the hyper-field and complete destruction of the ship's reactor (you're dead).
Now, set the universe up so that lasers are crazy-inefficient, physical weapons are usually standard, and explosives aren't dangerous enough to warrant carrying most of the time. Suddenly, you have a setting where fighters aren't a terrible idea (they are the only ones that can use hyperspace constantly, and can drop explosives/high V projectiles onto other vessels), regular ships dominate the standard field and can defend against other threats if designed properly (to use PD lasers against fighters and explosives against subs), and stealth ships are not for just anybody but are useful (sub-space 'subs' getting exploded easily at close range, but are restricted to close-range fighting).
Well, in vanilla BR we're going to want fighters to be somewhat viable even if they are just used as a way to distract an enemy's fire. If you'd ever played Tie Fighter than you have a general idea on what we're looking at for fighter capabilities (with the right anti-capital ship armaments), and as Strait Raider points out: it's all subject to gameplay mechanics.
Bare minimum: A lone fighter should not be able to take out a capital ship. A full attack wing will be the goal.
Seems that the best way to do this is to size your weapons/ships so that X fighters can take on the 'ideal' capital ship.