What would a mega job be anyway? The current job with special new fancy skills? How would it be different than the current job system being a boosted version of the basic class.

They won't add new "mega job" they'll just expand on the current job with new skills and maybe new traits (because let's face it, if they add 5 skills every expansion we will soon need 4 full bars so new traits would be welcome).
They've said they will also add as many job as possible. I doubt they will ever create something as "spec" (like, Frost Black Mage, Fire Black Mage). In itself a job could be considered as a spec. Like the Summoner being the DPS spec of the Arcanist and the SCholar the Healing spec. It is highly probable that they will at some point expand on this by creating new job sharing the same main theme. The Astromancer has a lot of "Time related" skills, hence the Time Mage. Why not the Templar (Bravely default) as DPS job of GLadiator. Basically, there's a limit to what they can create new in term of "theme". But there's less limit to exploit the same theme with different facets.

If they ever add something that could be considered a spec, it would probably be having to choose for between two set of skills to make people fell different. But really I highly doubt this will ever happen. They would probably instead give us some kind of new skill changing the way we play. (Like Nocturnal and Diurnal Stance, Buff vs healing fairy etc).

I don't remember where but I've read that Yoshida said they might even remove the "class" system in itself because they think it's kinda irrelevant right now. It is cool lorewise but there's very little point in it considering all future job may not have a basic new class. If they don't touch them we will probably not have a new class UNLESS they introduce a new Arcanist like class that will split into two different roles. (Let say they introduce the Blade Master become either a Spell Blade or a Samurai, the Blade Master would have all the basic spells and then from lv 30 you would expand on the job new skills). Right now, the only job having a class that makes sens is the Arcanist, all other class are pointless as they directly evolve in their job. They could simply make you start as Occultist and when you reach lv 30 the Black Mage chain quest start (without the lv15 archer requirement) and your class icon would change with the fireball one. The Job crystal would simply be removed and stay as a lore item.

They could however at some point expand the job crystal thing by giving us some kind of option for "spec" (as said before). Like, lv 65 you get a quest giving you a choice, would you expande your power regarding X side of the job or the other side. Each quest path would grant you access to different lv 65, 67, 70 skills. This would be a way to create "spec" among FF14 players. You could simply get the other skills by seeing your job trainer and starting the other chain quest to get the other skills (so you do not miss the lore), once all quest are done you can simply choose which set of spells you would use. Those 2-3 new spells wouldn't change your role but would be close to the current WoW talent. Like, they're not mandatory spells, they're situational spells. You get to use the one you prefer to play with.

My 2cents.