Sonic Universe (A Sonic comic series) has officially announced they will continue where the Sonic Underground series left off and will even give it A proper ending! More info here!

http://www.sonicretro.org/2012/10/sonic-underground-to-be-finished-in-comic-form/#comments

http://www.sonicstadium.org/2012/10/sonic-underground-getting-epilogue-in-sonic-universe-50/

http://www.bumbleking.com/forum/viewtopic.php?f=4&t=6219&sid=a2e97f763fc48459ce3c05a0a7e599ac