>>531637947
If you're interested in retail just choose whichever expansion looks the most interesting to you and do that, people may still be doing dungeons for that era, when you queue for a dungeon you can either choose the timeline you're currently in or dragonflight, if the queue is longer than 5-10 minutes you'll get a popup asking if you want to continue searching only for that era dungeons or if you want to expand your search to all dungeons, so basically if you like cata continue with cata and at 70 you'll get given the current expansion intro quest automatically

If you want to play classic, era/anniversary are kind of hard to break in to right now unless you play at peak hours, cata classic only has 4 days left until MoP comes out its more active than anniversary at this point though and MoP has more hype than cata so it will probably maintain a healthy population assuming you're interested in that, expect it to crater when MoP is done and WoD comes out though