I enjoyed the last few dungeons BECAUSE they were easy. I really hate stressing and finding things too difficult. Getting to that stage and passing those dungeons felt like the game was rewarding me for working hard with everything else they threw at me!

There's lots more hard dungeons you can unlock now - as well as more primals being added, new Ninja class coming soon... Try out some other jobs, collect Tomestones, do hunts, you can do Crystal Tower/Syracus Tower and looooads of other things. Bahamut as was mentioned. The story is constantly being continued with the release of each patch so in between the patches just level a new class or something