Alright, I have an idea for a boss for Underground Zone.
Sorry in advance for the large number of images.
Here we go:
Sonic enters boss arena and Robotnik drops down and starts pacing across the top of the stage akin to HCZ boss.
At this point, he begins being a typical bomb-dropping boss ala SLZ/CNZ/HCZ. The bombs explode and scatter little fireballs.
After 3-4 bomb drops, he'll stop and slam down into the ground trying to impale/crush Sonic. If Sonic is able to dodge, Robotnik will get stuck in the ground for a moment where sonic can attack (ala SYZ).
After 3 hits, Robotnik will rise to the top of the level and drill through the ceiling, exiting the area, presumably fleeing.
Then, the player hears drilling and the level starts to shake and rumble, then water starts pouring out of the hole in the ceiling.
Robotnik returns to the flooded arena, now having exchanged his side-drills for propellers.
Robotnik then moves up and down on the side of the arena for a few seconds (this is his vulverable point), then stops at Sonic's Y location. He then lunges forward at unpleasant speed attempting to drill sonic.
Robotnik, after reaching the other side after dashing, will return to the top of the level, and continue his bombing ritual, returning to the lower parts of the level to lunge after 3-4 bomb drops.
At this point, the fight is more or less like it was before it flooded, except Robotnik lunges from the side instead of dropping down. However, the difficulty has increased as the Sonic and the falling bombs both move slower, but Robotnik still maintains full speed because of his propellers.
But McGuirk, how does Sonic get air?
The bombs! This will add a fair amount of difficulty, as Sonic not only has to dodge the fireballs, but also has to get back to the center in time to get an air bubble. If this is deemed too difficult, instead of producing 1 bubble, the bombs could crack the floor and cause one of those little bubble dispenseing air leaks from the water levels to appear. If this was the case, the bubble leak would persist until another bomb was dropped.
As for what happens after 6 hits, I have yet to come up with any particularly good ideas.
Now for Audience Participation!
Constructive Criticism begins now! But be gentle:P