Changes were also made to levelling in earlier content with Legion's 7.3.5 patch. The level-scaling tech introduced in Legion not only be continued in the new continents of Kul Tiras and Zandalar, but was also be applied to content from prior expansions, allowing larger level brackets for lower level zones. Further, as previous expansions are now included in the base game, the level ranges for those expansions are now broadened, allowing players to spend more time leveling in preferred expansions and avoid others entirely. Some examples include a zone like Westfall having its level bracket (at 10–15 as of Legion) increased to 10–60, whilst continents such as Outland and Northrend sharing a 60–80 level bracket. The aim of this change is to encourage more player choice whilst levelling and to allow players to experience the full story of a particular zone without overlevelling the relevant quests.
Professions skills are split between expansions now, you'll have a separate skill bar for each expansion. This means you can level the new Battle for Azeroth skill tier without putting any point into any of the previous expansions tiers, you don't have to level Leatherworking with older materials to be able to craft the new BfA items. You can read more about this in my BfA Profession overview.
While the proc is random the law of large numbers will mean that over a large enough sample size your actual yield will be the same as the average proc rate. If you transmute 10 times then you can't rely on the 20% proc rate, once you pass 1000 though, you're average will start approaching the true rate of 1.2 bars per transmute. If you are in this for the long haul it then makes a lot of sense to use the average proc rate to calculate your profits, especially considering the fact that all the items involved sell really well.
Mythic+ rewards will also go up by 30 item levels; however, during the first week of Season 2, rewards from end-of-run chests will be capped at Mythic 6 quality (item level 385), and the chest you open next week will contain Season 1 rewards if you do a Mythic+ run this week. (So, for example, if you do a Mythic 10 during the final week of Season 1, you’ll get a Season 1 item level 380 piece and 60 Residuum from your first Season 2 chest.)