It's kind of a battle-pass situation now. Take a look at the meta decks and see what you might want to play (I recommend Vicious Syndicate's Data Reaper report), then take a look at the pre-built decks in the shop. Depending on what's available, you may be able to target cards you want to start building up your collection again.
As far as arena goes, I believe that still has the most potential to get a return on your investment. Whether that plays out for you or not depends on your skill and understanding of the meta since Arena cards rotate regularly.
You can get your monthly rank chest in a variety of game modes, now. If the Standard meta isn't friendly to you, you might give Wild or Twist a shot.
That's all I can think of right now. I don't really know first hand what the new player experience is like right now, but if I remember right, you should be offered a 'welcome back' package that includes a pre-built deck. Some are probably better than others but I have no idea what they're offering at the moment.