Which job seems the most 'relevant' to the main storyline?

[Tenkuu]

It has aleready being said we will have 3 new jobs

Actually, just recently Yoshi-P said in an interview that getting 3 new jobs out for 3.0 took a lot out of his team, and that it might be unreasonable to expect this number from every expansion. That seems to give the indication that there might be less than 3 in 4.0.