you can use oringinal cartridges on top
of the 80 games pre loaded be found on
original games can be found on
In regards of cartoon themed games, Ben 10 came out years after the original Sega Genesis was discontinued in 1997. But in the case of Batman, there are various games featuring him (most of them are based on the movies such as the 1989 Batman film, Batman Returns and even Batman Forever).
As for adding games, the fun thing is that it does work with actual Sega Genesis cartridges which can be found in secondhand stores like Goodwill, not to mention classic game stores, and Flea Markets. There are some games which will not work however such as OutRunners (due to its processor being too powerful for this system, along with Virtua Racing (due to the SVP chip which provides it the power to produce the polygon characters).