I think having to change spells depending on your Haste level is a good thing. This is what MM Hunters did mid/late Tier12 with Arcane vs Aimed. When you are below a certain Haste threshold, dumping Focus with Arcane Shot is better, above that threshold dumping with Aimed is better (or was, maybe they changed it). Now, in mid/late Tier12, you were at about that level were you might be above that threshold with procs and below it without procs. It is a dynamic rotation and I can live with a similar scenario for Demonology, where you use SF in Meta above a certain Haste threshold and Demonic Slash below that threshold. However, I would feel really weird dropping Meta when a certain Trinket procs to have more SF casts and thereby more Fury. I can't say if that is good or bad, just weird, as someone pointed out.
However, something else bothers me. With Demonic Slash being instant, we won't benefit from Bloodlust that much in Meta. I can see this as being a problem for a couple of reasons. First of, it feels wrong to just not be stronger with Bloodlust. "Hey, here is that awesome buff that makes you bigger and gives everyone additional DPS only not you!" Secondly, it would mean that for difficult encounters where you might use BL to push through a particular phase faster we won't be able to really contribute all that much.
About Demonic Gateway, I really like that spell as an option, Depending on the bossfight it might be really useful. I would however suggest that you don't just walk into it but instead have to click it, so you don't accidently walk into it. This might sound stupid at first glance, but you basically create an additional "Void Zone" for your Raid to not stand in. If space is an issue because you have to dodge some Firepatches or whatever, you now have to dodge the teleporter as well (assuming you have a plan for the portal in the fight). I would love to post this on the US forums, but I can't. Would be glad if somebody could repost it (Jessicka maybe?). Thanks in advance.