So, I've seen more than a few people (and I agree) saying that Arti needs some more oomph for epic levels. What they could REALLY use is a destiny that's better geared toward artificer. But adding another destiny now doesn't seem like the best idea ever, so here's my suggestion:
Rework Fatesinger (which is weak anyway) to work well for both bards and artis. Here's how I think it could be done well:
Remove all the bonus songs and the granting of Inspire Courage from Fatesinger. Bards don't need that many bonus songs, and gearing the abilities of the tree to work off songs doesn't work very well. Plus that Inspire Courage grant is annoying as **** because once you get it, every time you spend a single point in ANY DANG TREE it kicks Inspire Courage off your quick bar. So get rid of that nonsense--an epic destiny shouldn't be so geared toward something that's a feature of a single class, anyway.
While we're on the subject, Music of the Spider Queen is broken--it initially fascinates vermin BUT they seem to be flagging as immune when it comes time for them to roll their save against it so they break in like, 3 seconds. Every time. Regardless of your skill. It's really annoying.
Add more spell points grants to the innate abilities, and also a good portion of ranged/melee power.
Make Reign work off spell points and make it affected by spellpower.
Instead of strength/charisma make it charisma/int. Strength-based bards aren't going to use this tree, anyway, they're going to use DC or LD or FotW.
Rework most of the abilities so that they are specialist buffs that use spell points instead of stacking on Inspire Courage (or maybe make them multiselectors so you can pick a buff OR a song thing). Dirge is terribad. Fix it. You can't have multiple Echoes of the Ancestors modes active so they should all be on the same ability multiselector instead of taking up so dang much space in the middle of the tree. Replace at least one of them with a runearm-buffing ability. Decrease the cooldown on Turn of the Tide and make it work on bosses.
I only have four suggestions for Shadowdancer:
1. take that godawful craptastic useless Shadow Lance ability and just make it "your attacks and spells gain X% chance to blind enemies on hit" plus a tiny amount of unholy damage. Why would this be good for arties? Because blinding enemies gives them a much better chance to get their sneak attack.
2. Add increased ranged sneak attack distance to the innate abilities, ending with basically "you're always in range for a sneak attack". That alone will dramatically improve this tree and add a lot of versatility to it for your crossbow rogues/arties. Heck, maybe even add something SPECIFICALLY crossbow-related.
3. The Dark Imbuement Epic Moment is ******. 2d6 unholy damage? Seriously? Increased melee range? Huh? It should be something like 10d6 extra sneak attack damage and some kind of Ultimate Vorpal where you auto-kill everything on a 20 regardless of HP unless it makes an assassination-based saving throw.
4. Make executioner's strike/shot INT based, and make it 100% if they fail the save instead of 35%. The save DC is awful anyway, why in the world should it be only a 35% chance of going off. Oh, and make stuff that modifies assassinate DC work with this as well.
And here's my (not very fleshed-out) idea for the 3rd Arti tree
Make Eldritch Knight the 3rd arti tree. Make Eldritch Strike a multiselector with a ranged version. Make sure the "spellsword" imbues work on ranged weapons if they don't already (or make them multiselectors as well).
I would say "Arcane Archer" except that all that stuff doesn't work with crossbow bolts. Which is lame.