Merging Heavy Thrust into Impulse drive definitely makes more sense than the other way around. Less editing involved. What I'd like to see SE add in return is a dynamic finisher to DRG's combos. One that you could use as your 4th, 5th, or 6th skill without losing your average DPS, while being a true finisher. They'd likely implement a dynamic potency on it like they did with Fang and Claw/Wheeling Thrust, but make it close to Full Thrust's potency (lower on 4th, higher on 6th), to balance it overall regardless of where it was used. That way a DRG could end their combo when they need to. As long as it didn't have a positional it would be great. It would allow you to save True North for when you really need it once you got used to a fight.
Flare has been used in BLM openers before too. The reason it isn't now is because 1) Your MP doesn't allow for it under its minimum cost, and 2) You'd be giving up a Fire IV for it which thanks to the extra potency buffs just isn't worth it on single bosses anymore. It breaks ahead on 2 targets however so by all means use it to finish off a rotation if it lines up with somewhere you'd want to AoE, especially if you have a Foul ready to recover the MP tick. I don't think giving Freeze a trait to allow it to be used at 0 MP would really change that all too much. More likely what'll happen to facilitate new skills though is Blizzard II and Fire II will be replaced by Flare/Freeze entirely in that case, since they could bake that trait into Freeze itself as part of a few tweaks to it.
As for Machinist, it needs Gauss Barrel to be moved off of the hotbar, not Hot Shot. It could easily be reworked into a trait and lose nothing. Hot Shot however can be reworked to give Piercing and also act as a massive heat generator to counteract Cooldown, which would free up Barrel Stabilizer as a recovery tool, something that MCH needs badly. They also need Flamethrower, Overheat and Wildfire to be looked at. I don't know if widening the windows on the latter two would be the best idea. I think reworking them both into a stack-based buff and debuff each would be better overall, especially if they only affected weaponskills. Same with Flamethrower to be honest, just make it instantly max your heat instead of the ticks. If the overheat window was reworked into a stack buff, then using Flamethrower for AoE wouldn't feel bad at all either.