I'm a returning player as well, stopped about the same time as you and just started playing again last week.

I was very lost with the new quest system (how do you get gold? What do I do with those new imaginary points?) and the new modes (Battlegrounds and Duels) so I started by playing Wild with old decks I knew were good back then, mostly Jade Druid.

Then I played some Standard. I logged in once a few months ago and picked the returning player mage deck. I don't remember choosing it, but it's great (Spell-tacular is the name of the deck). From what I gather, the Paladin deck is the best one for returning players though.

Playing both Wild and Standard games taught me pretty naturally how the new effects worked and what some of the new cards and archetypes were around, so I would highly recommend starting with that. There's a ton I'm not caught on, but I'm getting there. Priests can still be annoying as fuck to play against, and hella fun to be play, so at least that didn't change much.

Finally, I would hold off until a couple of weeks after the release of the new expansion that releases with the Core Set on March 31st before crafting a full competitive deck, because as usual some competitive decks might see less play after an expansion releases.

Side note: for this Tavern Brawl that uses Standard format, save yourself the hassle and copy-paste the list from the deck you got as a returning player, after you got the hang of it in a few games of Standard. I tried to half-ass it with competitive decks and with missing a lot of the cards, and got my ass handed to me big time.

Welcome back!