One possible line up would be to use the G1 character Nachlicht as Luna's son via an affair with Sambra. The idea of the two having been a hot and heavy couple has been put out there by a few individuals, with the theory that Sombra meddled in dark forbidden magic in an effort to become an immortal alicorn. Night Light Sparkle, Twilight's father, having a strong resemblance to Nachlicht could then be cast as his son, or grandson. Keeping in mind we really don't know how long ponies would normally live, but it's fair to assume those with a lot of magic live a lot longer then other ponies. Granted the idea discounts the comics, but I'd think it'd make a lot more sense then saying Luna became Nightmare moon simply because she was jealous of Celestia. It could also explain why Nightmare moon went so easy on Twilight and the others when she could have used her full power to stop them. Subsequently when I look at the picture of Luna snuggling and grooming Twilight, it's that parent/grand parent - child/grandchild relationship I see.