Angel Island Zone
Once a peaceful floating island in the sky housing the Master Emerald, Angel Island was sent crashing down into the ocean by Dr. Eggman after the events of Sonic 2. Once Sonic and Tails found their way to Angel Island, they were interrupted by Knuckles, who, under the suggestion of Dr. Eggman, took the Chaos Emeralds and ran off, starting Sonic's journey in the iconic Angel Island Zone. After the events of Sonic 3K, of course, Angel Island was returned to the sky and all was well.
Fast forward to Sonic Mania Plus' Encore Mode, and the island is back in the sea! ...For some reason. Regardless, just like its original incarnation, Mania's Angel Island Zone is a tropical jungle with greenery just about everywhere you look, but this time it's where Encore Mode starts, being a little mini act that only extends to where the original had its iconic Fire Breath encounter. Instead of the island being bombed, however, Sonic finds a capsule containing two then-obscure characters, Mighty the Armadillo and Ray the Flying Squirrel! After rescuing one of them (the other turned out to be the Heavy Magician in disguise, the sneaky girl), Sonic & Co. are sent back to Green Hill Zone through the power of the ever-elusive Phantom Ruby to do everything again with a new coat of paint!
The Combi Confiner
That capsule that I mentioned in the opening bares a striking resemblance to the Combi Confiner, a machine that has its origins in Chaotix, better known as that one spin-off that has Knuckles in it. In that game, the Combi Confiner is first seen during the opening with Espio the Chameleon inside, who quickly joins Knuckles after being freed, much like either Mighty or Ray in Mania. This is here as a nod to Mighty's most popular appearance (besides Mania), being one of the playable characters in Chaotix.
todo: add comparison images blah blah you know the drill i cant be bothered to go get screenshots right now
High Five!
Likewise, the little jingle that plays when you choose who will be your partner in Mania is a remix of the jingle that plays when you get your partner in Chaotix.
todo: add audio