In FFXIV, I would like a system with one, or perhaps a few, quick level cap increases but where a stable level cap is eventually reached. I thought the FFXI system bringing the level cap from 50 to 75 was done well. I think players were aware those changes were happening relatively quickly and none of the gear before then was overly time consuming to acquire.

The issue with level cap increases, as the OP pointed out, isn't with the actual level of the players, it's with the associated impact on gear acquired over the years. I prefer new/better gear added to MMOs over a relatively long time. Gear that is more easily attained when your character (and linkshell's players) are outfitted with up-to-date gear. That way, your previous gear doesn't feel useless but at the same time providing new challenges and goals to strive for. I think FFXI could have done a better job of introducing gear that replaced or upgraded enduring elite pieces. Relics, ridills, adaberks, etc. were far too good for far too long and any sense of new achievement was lost once those were attained because incoming gear was largely inferior or, at best, a lateral move (usually inferior).

On the other extreme is the mindset of an MMO like WoW. It's not that the level cap increases come too fast but that the new gear absolutely smashes previous sets. The most common rank 71-79 gear introduced after a level cap increase was better than the most elite rank 70 gear players spent a year or more getting before that same level cap increase. That is insanely frustrating. Essentially, you end up skipping over mountains of content all for the newly introduced expansion. And that the newest of players were right on par with the most seasoned veterans after an expansion because they all quickly grabbed the better, common gear and then worked towards the elite pieces. Maybe that's what designers and developers are striving for, but it's not to my taste.

So while both system have their ups and downs, I would like to see a system closer resembling FFXI's system (but improving upon the introduction of new gear that actually outperforms old gear). That is, a relatively long and stable rise to a set level cap, where gear introduced is a modest improvement on already-existing elite pieces. And that the new content is tackled more efficiently with characters that have mastered previous content (and are appropriately geared because of that).