I would like to see an energy cost increase and automatically reload (because it would make them very annoying otherwise). Something like 1.5-2x energy cost of firing should be sufficient (or flat +500-+1000 energy cost or something).

Some? They'd become unusable again.

Wow, are you not happy that getting these artifacts isn't a disappointment anymore?

They aren't guaranteed to return, they aren't guaranteed to auto-reload when they do return, and you aren't guaranteed to find them on a particular game. I think that balances their power very well.

I do think they are too good now, yes. They technically aren't guaranteed any of those things, sure, but in practice they pretty much are for any given fight. I don't think returning to inventory is the right way to go, but I think an energy cost for reloading them would make a lot of sense.

If you think they're a bit over powered now why not just decrease the damage a bit? Simplest solution?

Reducing power is a simplistic solution that doesn't really solve any problem. If there's no trade-off (i.e. increased energy cost), they're either good enough to use or not. There's no real middle ground. Think of it in terms of tremendous blow and it will make much more sense what you should go for.

I definitely do not agree with this. Both projectiles already have problems returning from time to time. If this happens when you're clearing a tension room or a vault, you're screwed unless you have plenty of other arrows/quarrels.
I feel that the current setup isn't OP, either of the projectiles has to be obtained first and then several other conditions need to be met to use those effectively.
They are artifacts after all, not your regular "of returning" missiles. Maybe they have been created so that they return exactly to their point of origin, reloaded in the archer's weapon, ready to fire.
In some distant Ancardian past, an archer that was at the same time a mage was annoyed that he kept wasting arrows and instead designed True Aim to counter that tedious part and simply have it return after use, that would make sense to me.
I think that the energy cost should stay, as well as their returning mechanism but perhaps damage could be toned down a bit. That way you're not making it a spamfest of useless messages and also do not touch the energy cost.

This is no argument. I remember posting several RFEs that targeted realism and balance but most have been downvoted, citing no need of realism in a fantasy game.

All returning missiles should have increased energy cost.

They should not.
Your character isn't the one performing the returning as it's done magically by the artifact item itself without your conscious thought or action.
As it's done automatically, the arrow/quarrel returns during the process of you drawing your bow or your crossbow (which typically takes energy points as reflected by archer's class powers).

When you have finally drawn either, the projectile completes its return cycle and places itself in the firing position, just ahead of the bowstring and you release the accumulated energy to shoot it one more time. Rinse and repeat.
This is why dexterity is so important as it determines your ability to time shooting with the returning.

This is how I imagine it works, I think it makes sense and so I'm perfectly fine with the current mechanism.

So, you say the returning is instantaneous? If not, range of target should maybe also increase action cost of missile returning missile use.

If that were the case you would have to also consider the flight time of arrows when shooting distant targets (10+ squares).
Maybe faster monsters should be allowed to move between the moment of the projectile leaving your weapon and the missile hitting its target, if it's so far away?
I think this would be too much attention to detail, though. Simplification works better here, it streamlines the gameplay.
Arrows/quarrels returning take the same amount of time as hitting their target in the first place, which is to say a fraction of a second up to perhaps a whole second.
That is a really insignificant amount of time, hence I don't see how the process should consume more energy points.
Throwing returning weapons like rune-covered trident and some other stuff, doesn't require more energy from you just because it's returning, which is important especially when you consider their size, weight and cumbersome design (when used as projectiles).
Arrow/quarrel is a small, light, somewhat aerodynamic object, I think its current in-game behavior represents this very well.

I am not suggesting massive amounts of extra energy cost, just enough to reduce their power a bit.

They don't need to have their power reduced, they finally work well and are useful as artifacts should be. Any nerfing would once again make them even less effective than normal arrows of slaying. Already when I face demons or powerful undead or giants, I often switch from true aim or thunderstroke to projectiles of appropriate slaying because in the end they can often be more effective. Returning artifact missiles should be left as they are.

I've only found "True Aim", for me it was too powerful. One hitting most things in the game with it. I'm talking greater blue wyrms, stone grues, greater earth elementals. Greater water dragon, ancient karmic wyrm, ancient blue wyrm in a handful of shots. Keriax was the only time it did not return, otherwise it would been too.

It is too powerful for an artifact now, it was fine when it did not return. I used it then, because it was awesome. Anyone that wasn't using it in its original form was missing out on a great artifact missile.

I haven't had the chance to test Thunderstroke and True Aim in the new prereleases so I don't have an opinion on whether they are OP or not (although I seriously doubt that they are more OP than, say, Executor or Vanquisher, and it's more original to have a couple of OP missiles in the game than more OP melee weapons).

However, I oppose any solution that requires the player to go through the hassle of reloading them again. The new modifications to missiles go in the direction of making them less tedious, and that's the right direction.

I have no idea how you have managed to achieve such a feat, I had two elven, high dx (50+) chars obtain true aim AND sun's messenger and at no point in the game was I able to one-shoot any of the monsters you listed, and even weaker ones took 2-3 shots.

Well, melee doesn't work at range where missiles do.

What if the energy cost for reloading a returning missile depended on tactics? This could give the player a tradeoff option between power and energy cost, and if the cost is kept reasonably small (comparatively), it can help to tone down the missiles a bit without ruining them.

An archer trying to clear D:50 with thunderstroke was fairly laborious and had to be backed up with standard missiles even though I had the rune covered trident as a secondary weapon as you could pretty much guarantee the moment I fired off any returning weapon into a crowded room it would fail to come back. The only things being single-hit-kills were normal orcs and gobbos, all the chaos servants warriors and wizards were taking a couple of shots minimum, ignoring misses.