Best build with all specializations for Rogue?

What do you mean by "better"? Peak damage? Sustained damage? Damage rate? Fun?

It's easy to get an OP build. Like capn said, Flask of Flame + Elemental Mines would be pretty broken. Flask of Flame + Hidden Blades would be pretty sick also.

It's kind of hard to beat just the vanilla Archer Artificer for OP damage.  Certainly with Mark of Death + Hail of Arrows + Leaping Shot, since that's just a multiplier on the existing Artificer wombo-combo. Mark of Death + Thousand Cuts might give it a run for the money and works better on small targets.

For DW Rogue, maybe some kind of spamming loop that uses Flask of Flame to make MotD repeatable, but I think the short duration of Flask of Flame means you can't really get a good spam loop going. What's a good high damage DW rogue ability that has a short duration?

??? Not sure what you mean.

If you mean focus abilities don't work with Flask of Flame, I didn't mention anything about that.

If you literally mean specialization abilities, I thought you said we can pull from any specialization? So why should there be rules about what can work with what, given that we're already breaking a major rule? This is all hypothetical, yes?

Interesting.  Alchemist in MP has FoF and then Elemental Mines (from Artificer) and that works.  Wonder what all they altered for that mode.

From what I recall for ME3 all the powers essentially had a unique number, at least in single player.  Where it got a little whonky was that there were a few that were separate versions of the power but with the same name, and then some multiplayer DLC powers for whatever reason reused numbers that were already assigned.

For the most part this didn't matter as every kit in MP had three powers with different numbers, but if you were trying to mod to create custom classes you couldn't combine two with the same number.  Dunno if there is something similar going on with this game.