It makes sense
Mark Ruffalo is officially the only Avenger that doesn't get his own entertainment venture. When The Incredible Hulk movie dropped, Edward Norton was still playing Bruce Banner. Imagine if he had stayed in the MCU? Either way, Ruffalo picked up the role for The Avengers and has played The Hulk ever since. Captain America had a trilogy, Thor has a fourth film coming out, Iron-Man had a trilogy, and now Black Widow is getting a film and Hawkeye is getting a show. That leaves Ruffalo as the odd man out. Marvel will be bringing a She-Hulk series to Disney +, and since in the comics Hulk and She-Hulk are related, it makes sense to include Mark Ruffalo on the TV show.
Talking with People, Ruffalo was asked about Marvel’s She-Hulk series. “I don’t know,” he said when asked if he would make an appearance. “I’m supposed to go talk to [Kevin Feige] to see if there’s someplace [for me].” He was then asked if he would “want to” cameo in She-Hulk, or anywhere else in the MCU, and of course Ruffalo said yes. “Sure. Why not?," he responded. It appeared as if he was gingerly trying not to spoil anything. Do you want to see The Hulk in more MCU content?