Here are 6 tees from the Sega Shop in 2018 - 2019. They mostly have a classic theme, and 2 are a variant of each other. The first tee has a 16 bit pixel look, for flying Tails & Running Sonic sprites. They're on a piece of GHZ ground, with a big ring background. The shirt is a charcoal gray. This one is nice, as it gives the sprites 'something to do' and makes them nice & large to create a colorful scene.
The next shirt is a dark royal blue with the same design next to it, but charcoal instead. These 2 tees use the large "Victory Pose" end-screen art from the games where the character was large on the screen after you beat it. It has Sonic, Tails & Knuckles all in the 'big pixel' for the art, in a ring.

The next shirt is a mottled gray, with many modern characters. The top has Eggman & Shadow, and the bottom has Knuckles, Sonic, Tails & Amy. There are some thick red lines/triangles in the background. In the middle is the Sonic logo, but in Japanese. The last 2 shirts are also variants. One is gray, the other is black, and both have "1991" on them with some lines around it. Breaking up the year's large numbers are Sonic, Tails & Knuckles, all classic style kind of bunched up in the center. Each of these are 25 dollars in the Sega Shop.