I think it will be 2-3 new classes.
If they release Arcanist on its own, we will see a glut of ppl all leveling one class, and thus it will be very difficult to get PTs together. FFXI never released a job on its own.. they were always done in multiples for this reason.
However...
If they release a new class simultaneously with 2.0 (the final release - not beta), then there will be an influx of new players and it might work out ok..
But I still think we will see at least 2-3 new classes with the following priority: (
**The jobs listed are what I would personally like to see)
- Arcanist > Time Mage
- Scout > Thief
- Musketeer > Corsair/Gambler (or outside chance Red Mage)
After that people will want a pet class, so we'll get "Tamer > Beastmaster" or "Evoker > Summoner"