If they increase every 5 ranks, always something new to do, but its really how they implement the new content, and modify the existing content every time there is a cap increase. 6 months is a good time period, this keeps new content always coming in, and making sure the existing content takes at least 6 months to complete would make the game even better. A Player should never CAP everything out, there should always be something to strive for. Character progression is the reason most players play MMO's. Keep those new things coming, and players wont get bored.

- Forced Rank Caps for Existing Dungeons is important - Example: R30 Dungeons level synced to Rank 30 if you enter them
- New Dungeons (minimum 2-3) with new and exciting content and strategies
- Existing Dungeons drops/mobs modified/changed to keep old players wanting to re-do these dungeons every expansion.
- Make existing GOOD drops upgradable, enforcing that players will go back to those dungeons to get those drops to upgrade their weapons/armor. Make both the old and new dungeons play a part in that upgrade, and even possibly make some of those materials to upgrade "unique" so DOH cant just craft 100 of them at a time, and make them a fairly hard drop to get at the same time. This keeps existing weapons/armor still required, and doesn't just kill them off.
- Make new maps/areas to explore and new mobs to grind on to get those new skills and hone your new abilities before you go into those new dungeons
- Make earlier ranks easier to obtain to help catch up to the main group on new characters - Example: Add/Increase the bonuses for higher ranks, rank 1-24 now gets a leveling bonus to help get to the content to do, increase that 5 ranks every expansion, so 1-29 would have a bonus once we goto 55, then 1-34 once we goto 60. This helps new players/accounts/jobs get to higher ranked content.

Horizontal Progression = Short Term Game Killer
Vertical Progression done smart = Lifelong Ever Expanding Game (Do not stop at 100, keep going, keep creating, keep challenging)