Race: I'm definitely going elf for my first (and cannon) Inquisitor. Of course I will replay the game a couple of times to explore different outcomes and dialogues, but as a whole, I consider my first playthroughs to be cannon because of the element of surprise
I'm a bit sad I won't be able to replicate a Circle Mage Elf like origins, but oh well, gotta learn to be a Dalish now.
Gender: Depending on whether Dorian is gay or not, pretty much. Whichever gender allows me to bang that cutie will suffice, although I prefer roleplaying a female. If Dorian is not a romance option, who knows 
Class: Mage, forever mage, and only mage. I'm pretty sure I'm gonna go for Knight Enchanter, since it sounds the most similar to Arcane Warrior, plus I like to be the support/defender/initiator of my party, a true leader. Since both my cannon Warden and my cannon Hawke are mages, I have this dream of creating legendary mage-figures across Thedas, so hopefully by the time DA: 5 comes, NPCs will be like "Wow, them mages are so cool and legendary"
Companions: So far Sera, Dorian and Iron Bull or Blackwall I think will frequent my roster, but I will try to, eventually, bring along all companions, especially in areas where I think they can contribute (e.g. bringing Vivienne and Dorian to the Winter Palace because they are good at social manouevres)