We know that SOD2 has no permanent heroes, but actually it DOES. They are on the radio. Sasquatch, Lily Ritter, Twain and Lt. Meredith are such. How about a series of missions where favorite heroes call you on the radio for assistance, just like the random characters do. In this scenario these calls would happen rarely and randomly. The heroes of the first game would be NPC's and not controllable by the player, they are making cameo appearances for old times sake with a real mission and interactions. When your mission or issue with them is complete they go away just like random characters do in certain missions. My personal picks for these appearances would be Amelia Crasman, Sasquatch, Maya Torres, Lily Ritter etc.