On the topic of Mixmence, I feel that classic Mixmence is actually the way to go this gen. There isn't really much to outspeed at Base 100, except maybe Hydreigon or Haxorus. The ability to Roost is very useful too, since Mixmence should function more as a stall-breaker in my opinion.
I like being able to outspeed stuff like Haxorus, Hydreigon, and Darmanitan (and really, if you've invested enough speed to beat them, you may as well go all the way unless the remaining EVs have a specific purpose), but the Speed investment definitely depends on player preference and specific team needs. There are plenty of good benchmarks for Salamence to hit, depending on what you need it to outspeed.
I agree, Roost is almost mandatory when trying to break stall because of all the residual damage Salamence generally takes (Stealth Rock, Life Orb, and Sandstorm all do a number on it).
Mix LO DD also seems like it would be worth looking into. Lure in Skarm and OHKO it with Fire Blast, or Gliscor with DM, etc. It could make a nice partner for Excadrill, although sand would definitely not be doing Mence any favors.
I believe that there was a Quality Control discussion over that idea a while back. From what I remember, the general QC consensus was that you're trying to do the job of two sets with one, and thus failing to do either job well. I haven't tested this personally, so I can't really weigh in on how true that is.
I actually really like MixMence, but he does better with sun support over rain since Hydro Pump is ass. Ferro is probably MixMence's favorite partner though, since hazards are the key to making that set work.
I've found that Salamence does well both with Drought support and against teams using Drought support, since its Fire Blast becomes obscenely powerful and it can afford to run Flamethrower if you're using it with Drought support. It can also use that boosted Fire Blast, Brick Break or Earthquake, and STAB Dragon-type attacks to hammer Drought-based teams as long as you keep it in decent health via Roost or Wish. It's got some way to beat most Pokemon commonly seen on Drought-based teams, although it obviously can't beat all of them with just one set. However, it's a good failsafe if you struggle with Drought-based teams and has plenty of utility outside of Drought conditions.
Though I still wish Dragonite had access to Superpower with Multiscale, that'd throw it over the top as better than Salamence this generation.
I'll try not to get too far into theorymon here, but I disagree with this. Sure, Dragonite has a more powerful Fighting-type move, but it's weaker on the Special side, especially since running Life Orb with Multiscale is somewhat counterproductive. I'm pretty sure that Life Orb Brick Break gets the 2HKOs you need anyway. I can't think of any other role where Dragonite would run Superpower, but if I've missed something feel free to correct me. I'm definitely open to discussion here.
On a somewhat different subject, I believe that someone was asking about a Rotom-W lure, and some other people were complaining about Rotom-W in general. I've found that Heatran with Hidden Power Grass makes an excellent Rotom-W lure since it can outspeed and 2HKO. Timid Heatran can 2HKO even 252 HP / 0 SDef versions without a Life Orb. If you forego Hidden Power Ice for this, make sure that you have another way to deal with Dragon-types, either by running Dragon Pulse on Heatran or by having a different Pokemon to handle Dragon-types.