1. MCH rotation is simple and efficient but fixed. RNG should be added on the 1 2 3 buttons, because it's when it's the most boring.
BRD and DNC are complete RNGfest, it's okay for MCH to have some. It could be trivial like a chance that Gauss round gets refreshed by X seconds or a bit bigger for example BioBlaster has a chance to be freely used regardless of the Cooldown.
For example the shotgun we saw in the trailer could be a proc that allows the use of said shotgun.
Yes, I'm still coping that the shotgun will not be a GCD combo from spreadshot.
2. Mobility will change in 6.0, I believe Yoshida mentionned it multiple time in interviews. We can feel that in Diamond EX and the last savage raid tier, it's quite easy to maintain melee uptime.
In other words, there is a high chance that melees will play just like ranged 80% of the time. If SQEX removes positionals entirely (which I doubt but no impossible) there will be no differences between the two roles.
Casting works on MCH, it could probably on DNC and if BRD gets reworked it could be redesigned with casts in mind.
If 2 happens as the best choice and design for ranged, it's going to be quite the irony that it the best solutions was to redesign BRD, not MCH.