For the good of Lego? I don't know (seriously I can't even find a list of which themes are most profitable or anything). But for me PERSONALLY, I would basically end all themes I don't collect anything of and replace them so that I can have sets from basically all of my favorite franchises
So, keep: Ninjago, Harry Potter, Star Wars, and Marvel. End everything else.
Start new lines for (in no particular order, as many as possible): Return lord of the rings/the hobbit, return pirates of the caribbean, avatar: the last airbender, how to train your dragon (I would also accept a "Dreamworks" line that includes HTTYD, stranger things, guild wars 2 (probably the most obscure thing here but hey there are a few themes right now from video games), maybe supernatural but I can only think of a few ideas.
Things I think would be great, but probably just a few large collector sets: Supernatural, including CMF, sherlock, the office, etc.