Honestly, I think they just need to buff existing methods more.

Make FATES offer a lot more EXP. Extend EXP reward to A-Rank and S-Rank hunts. Increase the EXP of the 'super-FATEs' so people are more enticed to track those. Buff the EXP from the alliance raid series to be more in-line with the EXP gain from Aglaia, if not slightly more.

IMO, EXP gain should predominantly come from large-scale content and content that contains more players over, say, 4-man or 8-man content. I just feel like EXP being good specifically in dungeons (generally speaking), just dilutes the MMO factor.