I absolutely loved the Underground back in the Gen.4 games! I would go in there several hours a day to decorate and hunt for Fossils and neat items as well as checking out the latest Trading Items for my Base every day and to bury some Spheres :3
I really loved the concept about digging up your own Items and Fossils (if done wrong, better luck next time!) and so on. They should definintely bring this back (Secret Base in ORAS please make a return!). Only downside was that there was "limited space" for stuff in your Base so hopefully there comes a time when this will be eliminated :3
Actually, if I'm right, the connection you make with your friends in the Sinnoh Underground is done by a local wireless that will still be available after the shutdown :)
So it's still possible to play with your friends Underground and catch that Spiritomb when you have met 32 people :D