The devil is in the details -- how do you make an arcane trickster almost as strikey as a rogue and almost as controlley as a wizard, such that he overshadows neither but is still cool on his own?
Rogues' sneak attack is +2d6. It's situational, but after just a few levels, those situations seem to come up constantly. It's balanced somewhat by the fact that his attack powers have a lower damage die than the ranger and warlock powers. So going with that, one way to balance is to just drop the damage -- maybe it's only a +1d6 sneak attack, or even a +1d6 at-will similar to warlocks and rangers (coupled with the lower damage die, because the trickster would be dealing [W] damage with weak weapons). Another way might be to make the damage even less convenient than sneak attack. For example, maybe it takes an action to apply like the curse/quarry, but you can only apply it to someone you have combat advantage against. You could build the trickster's powers to have poor synergy with his striker bonus damage (many of the rogue's powers grant him combat advantage; maybe the trickster's powers don't). Or you could go the other way and give the trickster no striker bonus damage, but instead make his class powers high-[W], single-target effects.
The wizard has ranged area attacks that deal decent damage and inflict status effects. To make the arcane trickster an almost-wizard, maybe he only has close area powers, so he has to get up into the thick of things to use them (which is where he should be anyway, to do his stabbing). Maybe his area attacks don't target allies, which would be a slight advantage over the wizard. I think the idea I like best is that his area powers would deal very little damage compared to a wizard. This is sort of a hidden "switch" mechanic; if the trickster wants to control the battlefield, he's doing a bit less damage, but when he focuses on one target, he can pour on the damage, striker-style. A grease-type power seems like exactly the sort of power this hybrid would utilize.
Those are just some general suggestions of how I would approach the problem. I don't really have any more concrete ideas -- it sounds like you've put more thought into specific hybrid combos than I have. Hybrid classes will definitely require playtesting to ensure that you hit the balance between too good and too mediocre.
-- 77IM
Rogues' sneak attack is +2d6. It's situational, but after just a few levels, those situations seem to come up constantly. It's balanced somewhat by the fact that his attack powers have a lower damage die than the ranger and warlock powers. So going with that, one way to balance is to just drop the damage -- maybe it's only a +1d6 sneak attack, or even a +1d6 at-will similar to warlocks and rangers (coupled with the lower damage die, because the trickster would be dealing [W] damage with weak weapons). Another way might be to make the damage even less convenient than sneak attack. For example, maybe it takes an action to apply like the curse/quarry, but you can only apply it to someone you have combat advantage against. You could build the trickster's powers to have poor synergy with his striker bonus damage (many of the rogue's powers grant him combat advantage; maybe the trickster's powers don't). Or you could go the other way and give the trickster no striker bonus damage, but instead make his class powers high-[W], single-target effects.
The wizard has ranged area attacks that deal decent damage and inflict status effects. To make the arcane trickster an almost-wizard, maybe he only has close area powers, so he has to get up into the thick of things to use them (which is where he should be anyway, to do his stabbing). Maybe his area attacks don't target allies, which would be a slight advantage over the wizard. I think the idea I like best is that his area powers would deal very little damage compared to a wizard. This is sort of a hidden "switch" mechanic; if the trickster wants to control the battlefield, he's doing a bit less damage, but when he focuses on one target, he can pour on the damage, striker-style. A grease-type power seems like exactly the sort of power this hybrid would utilize.
Those are just some general suggestions of how I would approach the problem. I don't really have any more concrete ideas -- it sounds like you've put more thought into specific hybrid combos than I have. Hybrid classes will definitely require playtesting to ensure that you hit the balance between too good and too mediocre.
-- 77IM