Somehow incorporating the shooting in multiplayer modes into single player gameplay. Here's one idea.
Sonic reaches Dark City Zone. He finds that Eggman has already left for the Grand Eggship / Egg Rock, and has littered his city with inactive tanks and steel barriers. Now, Sonic can get through Dark City without destroying any of the steel barriers, though that will force him to weave through more dangerous areas. To break down the steel barriers, Sonic will have to commandeer Eggman's old tanks...
If Sonic jumps on top of a tank object, the player's view immediately changes to the first person perspective. The directional keys control where the tank slowly rolls, the jump key allows the player to control Sonic again (who is deposited back on top of the tank), and the left mouse button launches a blast that can destroy steel barriers or a wide range of enemies (aimming with a HUD crosshair) with the same effect as an explosion ring. Destroying certain steel barriers will reward the player with shortcuts, monitors, and emerald tokens. Furthermore, the player is invulnerable from harm while riding the tank (though it is still possible to be killed by falling off a bottomless pit).
That said, tanks have several weaknesses. First, the player is much slower while operating a tank. Secondly, the tank has a reload delay of about five seconds (although ammo is otherwise infinite), as well as limited energy (indicated by RELOAD and ENERGY bars on the lower-left of the HUD). Each tank starts out with 100 energy points, but one point is siphoned for each second the tank is in operation, like Super Sonic's rings. Furthermore, each enemy attack to the tank (contact with Crawlas, bullets from Pop-Up Turrents, bombs from Jetty-Syns, ect.) takes one energy point from the tank. Once a tank depletes its energy, it can no longer move or shoot, and Sonic is forced to find another tank or continue on foot. Tanks exited with remaining energy points will resume from the same amount should the tank be re-entered. There will be no way to replinish tanks' energy points aside from dying, which also puts all tanks back into their original positions.
So basically, the purpose of the tank is to navigate through areas while taking out large amounts of enemies to blast through steel barriers, which contain secrets, before the tank ceases its functions.
Because one of the main focuses of SRB2 is speed, I can't imagine there being more than maybe three tanks and three "tank obstacle courses" in a single act.