Ideally, with each new increase in level cap, it would be accompanied by new content AND new Classes and/or Jobs. That way, it's not only about getting levels 51-60 of your current Class, but also about possibly going back to get 1-60 of a new class, and going through lower level content as that Class if you want.

Just off the top of my head (so don't expect balanced classes or specifc items)...

1st Cap Raise:
  • Add Musketeer, Arcanist, & Fencer Classes.
  • Add Sniper, Scholar, & Red Mage Jobs.
  • Add more Level 30 content (quests, NMs, rare equipment).
  • Raise Level Cap to 60.
  • Add Level 60 content.

2nd Cap Raise:
  • Add Whip-User (Lasher), Tachi-user, & Mace-User Classes.
  • Add Summoner, Samurai, & Ninja Jobs.
  • Add more Level 40 content.
  • Raise the Level Cap to 70.
  • Add level 70 content.

Rinse and repeat every year or two for 5-10 years.

As long as the cap raise has:
  1. New Classes
  2. New Jobs
  3. Max Level Content (calcuated after raise)
  4. Mid Level Content (calculated after raise)

There shouldn't be a problem with veteran players racing to the cap, and thus making their old gear obsolete, as long as new Classes/Jobs/Content/Recipes are introduced that can make use of that gear at the same time.