Not new to rping, and I have a couple ideas that might be fun, both lean heavily on OAP but I can always be talked down to more reasonable ages if that isn't your cup of tea. Message me here or my main F-List account, the name there is Magus of Souls
I'm down to do either of these or work out an entirely different idea
To the Demon King's castle- Your typical fantasy setting, a group or duo of young female adventurers set off on a quest to defeat the Demon King. Only this journey doesn't take months or even a couple years, instead it takes decades, the party amasses power and over time has to find ways to more cleverly defeat their foes as they get older and weaker, until they eventually make it to the castle, many decades after they started their adventure as fresh faced travelers. This one obviously has more planning and can be more involved, but we can make it streamlined and more fast paced if you so desire.
A Magical Girl never retires- You ever wonder what happens to Magical Girls when they grow up? Do they hang up the wands and move on from saving the world? Well not all of them, some are bitter at the loss of their youth and spend their days looking for ways to reclaim it, and one in particular has found the perfect way. A particularly nasty curse that marks a young Magical Girl, slowly draining the youth from their bodies and right into the one who cast it. Sure it might seem good at the start, being a few years older can make you stronger, but as years go on it might be harder to keep up with your enemies, while the one who cast the curse is getting younger, and perhaps young enough to take it all from her victim in one fell swoop.