What seems to be the issue? You think the endgame is boring and not challenging? Maybe you should look to see how far you can go with a self made build? Polish it and push? You could always look for another game because to me it sounds as if you don't like the game. Also, if you're rank 30 now, then you'll be way, way down when the season ends. Top 30 now and stopping is like having a good start in Mario Cart and then just say you're done without finishing the race.
I'm so curious though. What is so good about D2 compared to D3 endgame wise? They both have the grind and while D3 have greater rifts, D2 has Baal runs for days? What do you do in endgame D2? I've been playing D2 like you play D3, never to it's fullest so I would not know. Please enlighten me!