You are looking at the problem wrong.
It may be too late to add a persistant BBEG at this point. But you can introduce one.
In practice, an NPC can't be expected to survive too many close shaves with the PC's before they'll eventually get them. Hoping for that often leads DMs astray into misapplication of narrative force, by introducing DMPCs, DM pets, and NPCs that enjoy plot protection. All of that are tools best left on the bench and are in my opinion beneath the dignity of a skillful veteren game master.
My general formula for introducing a reoccuring BBEG guy is the following:
1) Introduce a sympathetic likeable NPC in a setting in which the players aren't expecting to find BBEGs and in a role where they are ready to classify the NPC in some other way - quest provider, mentor, shopkeeper, ally, local ruler, etc. Genera savvy individuals may expect this, so introduce red herrings and mix the NPC among other NPCs who really are who they seem to be.
2) Repeatedly work this character into minor scenes.
3) Introduce mini-bosses. These are the BBEG's minions. Each has a certain number of 'clues' that reference the BBEG through an alternate name - correspondance with the BBEG or other minions that reference the BBEG, monologuing, death scenes, interrogation, overheard conversations, acts of worship of the BBEG, etc. This introduces the idea that there is this BBEG out there, the master mind behind all of the troubles that the PC's are working through. If the mini-boss manages to escape once or twice to become a reoccuring character of his own, well so much the better.
4) Build multiple settings for the BBEG that have a 'trap doors' built into them. These are ways you can gauratee the BBEG escapes once the PC's figure out that the BBEG is actually the Janitor from scene two and you've Scooby Doo'd them. Trap doors include obvious things like Teleport Portals, Chute Traps, Charmed Genii/Fiends that carry of the BBEG if he's in trouble, etc. Pretty much anything that the BBEG can use without taking a disruptable action (or an action at all) and which is very hard for the players to stop. You are building dungeons where the goal is to give the BBEG a chance to maniacly laugh or say, "Curse you, I've been foiled again.", and then get the heck out of dodge. Sometimes you have have trap doors that don't look like trap doors. There are a lot of spells that let a caster play games with the players and escape their powerful narrative punch - Projected Image, Programmed Illusion, Simlacrum, Clone, Improved Magic Mouth, etc. - all let the player interact with the NPC in a way that keeps the BBEG safe. Make sure you have multiple trap doors, and make sure you have adequate 'moats' to delay attack on the BBEG. Moats are things like minions, literal moats or other obstacles, traps, walls of force, glasssteel windows, etc. that separate the BBEG from his pursuers and help ensure the getaway. If you are counting on Teleport, and round 1 they hit the BBEG with something that prevents teleport, then your BBEG is probably NOT a reoccuring villain unless you've also planned a Chute Trap or One Way Secret Door that leads to a trap filled labyrinth that delays pursuit etc. If you don't have a sufficient moat, and they grapple the BBEG on round one, then this probably isn't a reoccuring villain. Races or monster types or classes that help with these powers of evasion are nice, but don't overrely on that. The BBEG's main weapon is as in stories, his ability to out think the PC's and stay one step ahead of them. That requires you to out think the PC's and stay one step ahead of them. Fortunately, you, like the supervillains of story, have virtually unlimited resources at your command a likely have players who'll accept that genera convention without question.
5) Keep in mind that there is no way to gaurantee a BBEG is going to reoccur. Plan for it, but don't plan on it. Have contingencies. The simpliest contingency is always, "The BBEG was actually working for someone else even more secretive and sinister."
