If you had the power to add one feature no questions asked, what would it be?

[kaynide]

...

Also a valid way to go; No complaints form me over horizontal progression or the ability to piecemeal your own custom combination of skills.

I really enjoyed how FFXI's jobs each had some weird niche it could do, and you literally couldn't do all content with all jobs (this was back when 75 was cap). Horizontal progression was AWESOME feeling, and it was really fun to find weird loopholes certain jobs could take advantage of. It's not quite what you're saying, but you could be any combination of any 2 jobs at any time (2nd was 1/2, so think like a Blackmage 90/Whitemage 45), with almost every ability of either job available. It was awesome, and sometimes made for bizarre meta.