My professional input, knowing nothing of your shop or location, and all respect intended: hosting DMing "lessons" using other systems like PbtA, SwoN, Starforged, Quest, Kids on Brooms/Bikes, etc. to hone narrative improv skills can often set a foundation of non-D&D preferences — especially when the aforementioned are super easy to get started, thus promoting a higher frequency of game sessions for your fledgling Storytellers/GMs. Hell, offering a free/discounted game room rental to the groups these GMs put together could be a great draw (& good optics) 🤓

Wildsea has been so fun and since its flavor is different than your standard DnD setting, but it still has a fantasy setting, I think it has pretty wide appeal. The style of game is different, and definitely uses different GM skills, but it's a great option imho. It can be run as a drop in/drop out pretty easily which is a plus for some groups. It will definitely not get the same reach as DnD, but the art is so inspiring it might sway some people if you're able to display it somewhere.