I'm going to use an example from Anarchy online which is an old sci-fi mmo I used to play. End game weapons and armor in anarchy online didn't have a best in slot until very late into the careers game instead they did what the video talked about. They made various different weapons of the same type that specilized in different ways of dealing damage and builds. Take hammers for example you had 1 hammer that had a very large damage range something like 100-1000 and the lowest armor class damage type so it could hit harder than other hammers but it was slower. It benefited from haste and base damage increases more then other because of how it was designed. Then you had a hammer with average stats lets say 400-400 damage range that was very steady on damage and debuffed defense rating for evasion. Then you would have another hammer around 300-500 damage range but lets say it had a higher crit mod and it did some other debuff.
That's basically how the weapons and armor played out end game items were gave varied builds more choice and dps different ways to put out damage or utility while keeping dps pretty much the same.
While end game raids had more versatility older items were made with special effects as well. for example the Enforcer profession (melee/tank) had a special tank armor gained form a level 30ish dungeon. This was the only armor in the game that gave an extra 1600 shield. Items with that unique flavor were carried around for a life time and made sure older content wasn't abandoned.
I can't see the problem with DDO doing the same with their new expansions. Say you make 3 raids 1 has an outstanding ice armor, one has a great acid armor, and one has an awesome fire armor. You couple this with other unique flavor items that fill niches in specific builds and you build around this. Now you have content that is constantly revisited because you made items that matter. Then when you release new content sure you can have better ice armor but instead of just increasing the values give it some type of rare mod. If raid 1 had ice armor that had a chance to freeze whoever hit it have raid 2 have ice armor that saps attack speed by a % etc.
In every MMo I've played I always had more fun especially where this gear choice is usually abundant( much like WoW pre 60). It's a lot more fun for me to say"well do i grab this sword that melts armor or do I grab the sword that let's me shadow step behind someone for a guaranteed sneak attack". You don't always have to get stronger number wise you can get stronger specialist wise. I find it gives the same feeling of advancement.