So working on that now.... the two plans I'm eye balling are...
Plan Chain Missile Fire Seed - A version of fire seeds (which I've fixed btw, it's physics were a mess. Like throwing beach balls. Bad.) Up to 3 missiles of Mini AOEs. So you get the 3 ambush procs but it's not really possible to just drag an entire monster pod into it, you'll be smacking like 3 mobs really hard. I think this basically what your suggesting?
Plan Chain Missile Chain Lightning - It'll hit 6 targets once but the AOE will be really big and you don't really need to aim it. The Missile will zip around over a pretty big area. Easy to use but only 1 proc of ambush per target, but it really easy to hit alot of targets.
-T
About the "Plan Chain Missile Fire Seed": but... that AoE shape (and number of hits) is already available. It'd be silly to have the same option at core3 as at core5, just a different damage type. Yeah they wouldn't share a cooldown, so people would take it, but it'd be boring during play. It would be *much* more interesting to have a different spell. So I wouldn't do this (unless you are planning to remove the option for Fire Seeds to be stolen).
The plan single-hitting Chain Missile Chain Lightning, on the other hand... if you make it 1d2+3 dmg
per level, it could work.
This Chain Lightning-like Missiles wouldn't exactly be competing with fireball and acid ball for sorcerers/wizards. For them, the damage would be pitiful, 9d6+27=58.5 is just a lot more than 6d2+18=27... not counting the difference in spellpower (and crit) yet. Still, at a third of the damage of their main element, it could still be an interesting backup for immune enemies maybe.
For wizards, especially archmage that get it for free, it would be decent to cast when they have nothing better to do. (at 1d2+3 vs 1d6+4 it's half the damage of Arcane Blast, and we all know that's nothing spectacular, so that goes to show it's certainly not too strong. but it would be better than it's current form and not completely useless).
Back to AT. Once a rogue gets Magical Ambush at level8, a rogue would deal 8d2+30+4d3=44 with it. Magic Missile at that point does 4d2+12+16d3=50dmg, but being single target, that sounds fair enough.
At 20 however, Magic Missile will have scaled to 5d2+15+70d6 = 267. Chain Lightning-like Missiles will be stuck at 20(?)d2+60+14d6=139 dmg.
That is acceptable. An SLA that you get at level 6 or 12 will usually not be relevant at cap (even if 'free', because it's an SLA).
I kinda like it. You could even buff it to 1d6+3 per level like a fireball without the balance completely breaking, but I like the 1d2+3 (pen and paper's 1d4+1 with ddo's cheater dice) pattern and I think you can balance around that.
However, it's true that AT also need a
true multi-hitting AoE for epic and cap play. The current option fire seeds in core5 already fills this purpose halfway decent (60d6 fire + 42d6 = 357 dmg at 20, for comparison), but the small AoE-size limits it's use.
I'd take Force Missile, remove it from the SLA-options, but keep it in the spell list, and buff the hell out of it. AoE (normal sized), 3 or 4 hits, but all hitting for just 1d2 per level or something, so that wizard/sorcs don't default to it. For AT however... once they get level 14, they'll have a nice delivery system for Magical Ambush (it would be kinda like the bludgeoning-portion only of meteor swarm. you could even use that animation, it's also an AoE multihitter).
The nice thing about this is that Fire Seeds at level18 isn't a strictly worse either; at 3d6 per level it's theoretically better, just a smaller AoE and fire-based. For those using their Stolen Spell that way, it's a nice extra option (doubling as small AoE, and another single target triple-hit). But in practice, Force Missiles will be better for an AT, and that's why it has a real spellpoint cost.
I'd put Dimension Door as lvl4 wiz/sorc theft option. Fits a trickster. Yes I know 99% of the tricksters will be in Shadowdancer destiny, but there are 5 more options to respec to then, it does make lvl12~22 more convenient, and it's just fitting. Alternative would be phantasmal killer, I guess.