If your friends are more interesting in dungeon #2, then you can still do that, and still get useful rewards, but you'll still have a reason to go check out dungeon #1 at some point in time as well (though maybe you'll need to join a Linkshell that's dedicated to doing that content instead of running it with your friends). Basically, all of your options are open all of the time. All of the gear and tokens and whatever are worth getting (assuming you don't already have them) rather than only the new gear. If you have all of the gear and the tokens that you need, then you've essentially "cleared" that content, and you never have to go back if you don't want to.
Bolded for emphasis. That right there is why I think this design is terrible.
It should never come down to me having to go to another guild and exploit them for the purposes of catching up. If I want to play with friends, then I should be able to play with them shortly after reaching endgame, rather than spending months exploiting strangers. This is the main reason why I think the emblem system in WoW works well; the emblems ensure that my friends will want to run things with me as I gear up, and I don't have to go and PUG nearly as much, or join some random nub guild and use them to get gear so I can join a better guild until I'm caught up to my friends.
Honestly, I didn't like that suggestion either, but not for just that sentence. In my mind, -all- endgame content should be -equally- open.
Consider heroics in WoW, except pretend that they don't drop gear. If you do DTK, you get badges. If you do Gun, you get badges. If you do UK or UP, you get badges. Maybe you don't like Oculus; you don't HAVE to do it. Maybe you don't like Nexus (I hated Nexus), you don't HAVE to do it.
Now let's reintroduce the gear, and pretend that no raids exist; endgame is JUST heroics.
In my mind an ideal system is where every event gives "points" or "tokens" or "badges" or "ham" or whatever you want to call it. IN ADDITION to the points/tokens (let's just call them points from here on out) you get from completing content, you ALSO get gear. I'm going to link part of an older post of my own:
Expansion zero (base game) introduces Castle and Dragon. Castle gives sword, helmet, breastplate, Dragon gives robe, staff, shield.
Expansion one introduces Volcano and Castle 2. Volcano gives sword b, gloves, boots. Castle 2 gives dagger, pants, necklace.
Expansion two introduces Dragon 2 and Beach. Dragon 2 gives ring, mace, shield b. Beach gives breastplate b, bow, staff b.
Castle and Castle 2 take 3 hours each. Completing either will reward you with 4000 "Awesome points".
Beach and Volcano take 1.5 hours to complete and rewards 1750 "Awesome points".
Dragon and Dragon 2 take 30 minutes each to complete and each reward 500 "Awesome points"
You can talk to an NPC in town (similar to the Dynamis/Limbus AF upgrade NPC or the badge/token vendors) and trade in your AP to buy everything from sword to staff b.
In my proposed solution, all content, old or new should offer three things:
1) It should be tuned to whatever the current level cap is, so that it provides a reasonable challenge and can't just be facerolled (like 6x99 walking through Nyzul or 10x80 doing Zul'Aman).
2) It should offer physical gear drops when challenges are completed or bosses are defeated.
3) It should offer points to all in attendance so that everyone, including those who didn't get gear, ALWAYS get points for attending ANY event.
Furthermore, there should be NPC vendors available who will exchange points (obtainable from any event, just like badges are obtainable from any heroic and any raid but the current tier) for any item available from any endgame event.
Don't like Dynamis? Do Salvage and use your points to buy AF2. Don't like Sky? Do Nyzul and use your points to buy Abjurations. Don't like ToC? Do Ulduar and use your points (badges) to buy T9.
The main advantage of a system like this is that id doesn't force you to do content you don't want to do to get gear you want, and it doesn't force you to do the same content over and over for one or two pieces of gear that just don't seem to drop or that you keep getting outlotted on.
Not only do you get to do the event you want to do (regardless of whether it's the newest, shiniest event or if it's the oldest event that you're just feeling nostalgic for or just happen to enjoy), you're GUARANTEED to get points that you can eventually put towards whatever item you want to get.
I see two downsides to this system, and here are proposed solutions
One is that as people get better gear, newer content will need to be made harder to remain a challenge, but making the content too hard will mean that freshly level capped players will have a difficult time overcoming the challenges if they are too hard. Proposed solution is to introduce two difficulties, a "regular" and a "hard" difficulty (like WoW hardmodes). Harder difficulty should allow players to receive higher drop rates (say 2-4 items per boss instead of 1-2) and more points than lower difficulty.
Two is that as more new content is released and the level cap is increased, gear will need to become substantially better in order to compensate for the new level cap, and endgame events will become "too easy" if everyone is +5 or +10 levels higher. My solution to this is twofold:
First, to address the issue of "too much endgame stuff", monitor frequency of participation and make note of events that players don't do. If few people do one or two events anymore, THEN phase them out. I know I said I don't like phasing out content, but if no one is doing it anymore, it's not because of the rewards (since the rewards are the same everywhere); it's because they don't LIKE it. If no one likes it and no one wants to do it, get rid of it. Keep the stuff everyone is still doing, and add a third difficulty for an interim period. Let's say you increased the cap from 75 to 80; in addition to "75" and "75 hard", you now also have an "80", with more points. Let it remain there for about 3 months (less or more, depending on how long it takes people to start doing the new difficulty option), and then phase it out so that just the "80" difficulty option remains.
Secondly, you're going to want more gear, and eventually you have such a thing as "too much" gear. Have the old content drop upgraded versions of the same gear at the new difficulty, and let the people who already have the old gear have the option of trading theirs in, plus a small amount of points (the difference in cost) to upgrade to the new equivalent.
Let's take an example. We have a dragon, we'll call him Fred. Fred drops a Sword with STR+10 that costs 5,000 points at a vendor and a robe with INT+10 that costs 7,500 points at a vendor. Fred is level 70 content, and awards 500 points because it takes 20 mins to get to him and 10 mins to kill him (30 total) with a group of 10x70.
Now, we have increased our level cap to 80. Level 80 Fred will take 20 mins to get to and 10 mins to kill for a group of 10x80. He drops a sword with STR+12 that costs 6,000 at a vendor, or you could trade in his old sword plus 1000 points. He also drops a robe that gives INT+12 and costs 9,000 points (or old robe + 1500 points). Level 80 Fred gives 600 points upon defeat. Level 70 Fred will still offer his old rewards for 3 months, and then will be removed.
This system has the benefit of allowing old work to not go to waste (as it can be upgraded toward the new model), AND it allows players to have options of what content they want to do, so no one feels "forced" into or out of content.
I don't care whether you want to label it "vertical" or "horizontal" or "diagonal" or "hokey pokey" or whatever; I think a system like this could appease fans of either system. It adds new gear without making old gear totally worthless, and it adds new content without forcing people to keep doing the old stuff OR forcing them to do the new stuff.