Well Team Fortress always only had 9 classes back with the original mod, and honestly, considering they haven't added any new characters in TF2 all these years, I doubt it would happen.
I dunno, I'm not Valve, but TF3 will probably never happen, since all they need these days are Dota 2 and CS:GO.
"Just for comparison Uncharted 4 was 20x bigger than Splatoon 2. This shows the huge difference between Sony's first-party games and Nintendo's first-party games."