One could design a system like conquest in WotG but have battle fields of varying levels caps and make it easy to access for new players. In a campaign battle, all a person has to do is walk up to a NPC and join in to earn points. You don't need to LFP. You just join in and leave when you wish to. Everyone obtains points... and should leave with something of value (but for all levels). A point system could allow you to trade in points for unique gear. The key is to make the battles have level caps of various levels (like in garrison) 20, 30, 40, 50 e.g. Let each tier of capped events give out different items that players of all levels need. That way players of all levels... level 20-99 can unite and play beside one another and have fun. They don't need to powerlevel to 99 to take part in a grand event with higher level players and feel left out of the game (just for being new).
A quick fix for CoP (it could be done in a single patch) is to let players decide which way they wish to enjoy the content. When they enter a CoP area, a window can pop open (much like it does when you do a BCNM) and ask if you wish to have a level cap placed on you or not when you enter the zone. Special rewards could be given to players who wish to do CoP the old way (with a level cap on). People who didn't wish to cap themselves can still rush by the content but gain no special reward. This option wouldn't be very hard to code and would allow people with both kinds of play styles to play CoP their own way.
However, before anything should be possible, a new system of storing gear is needed. A player should be able to store a set list of equipment, and retrieve it all with a single commant from a moogle. That way a higher level character could quickly grab a set of level 20 armor quickly if he wishes to take part in a level 20 event. If a player has to take a hours, hopping from mule to mule, to gear up for an event, it discourages players of higher level from doing them.
Another problem is that SE thought placing level 75+ monsters in zones right next (or 2 zones away) from starting zones would generate more traffic in those areas. That was a huge mistake. If anything it make the game less newbee friendly and just served to frustrate crucial new customers. It handed them a dose of confusion while making it near impossible to do low level quests at low level (like they were designed to do).
One of SE's first thing on their agenda would be to remove those monsters (as high level players level elsewhere anyways). They do more harm then good. At the very least move them to higher level but accessable areas like crawlers nest. By then players know how to evade mobs and have access to abilities to go undetected.
In summary, the following ideas, in this order might be a good step to fix the current problems that FFXI has.
#1 New player retention.
A) Add an offline tutorial to the game that explains battle mechanics to the new player. When I started playing in 2004, I got a manual with the game. Now, since the game is downloadable, it needs some form of tutorial to teach new people.
B) Remove all of the level 75+ monsters from low level areas. Don't have monsters of that difficulty 1 to 2 zones from starting towns. Place them in higher level areas, away from vital low level quests.
C) Design a new search system for players to find other players play with. It would allow a new player to get help from veteran players with a simple click of the mouse (the details for it I posted above). :3
#2 Experience Point Exploitation (Ways to slow down exp gain and enourage skill to gain it).
A) Remove abilities given from GoV and FoV and limit their exp boost and powers to once per Vanadiel day (like it was before). This should slow down EXP rates and turn focus from book burning to using the standard 6 person party method.
B) Make monsters gain a resistance to player AoE attacks when players use them seconds apart from each other. That would put an end to massive exp gains with little skill. That should stop players from sitting AFK in Gusgen Mines while other players use tons of AoE in a row to get a huge exp gain.
C) Restrict Abyssea to players 75+. With that restriction in place people won't sit and "leech" exp by doing nothing. Also lower some of the exp rewards there so players spread out and find more then one place to gain exp.
#3 Mid-level player retention
A) Add events like garrison or CoP (that I described above) and make it accessable by people of all levels. Have some from of useful reward for all levels for taking part in them. This will prepare new players for end game events. It will keep those new players from quitting the game due to frustration of not understanding group battle tatics.
B) Reinstate the level cap for all CoP zones. This will also give mid-level players something to do as they level their character. Giving players more things to do as they level, makes the leveling process less painful. Hand out exp for completing storyline missions, as well as non-repeatable quests.
#4 Storage and macro changes
A) In order for any of this to work, it has to be quick and easy for a higher level player to get his gear to take part in those level capped events.
B) Design a mannequin like template that allows players to scan players and store their gear in their inventory in that exact configuration. Allow them to retrieve it for free. If there is a cost, have it based on the character's level (much like chocobos are).
#5 The Gil Funnel
A) Reduce the amount of gil gained from chests in abyssea and from other sources that drops them in large amounts.
B) Add more gil sinks to remove the current inflation in the game. Design them in optional areas of the game so it doesn't force who don't have much gil unable to participate in common tasks.
In short, this will slow down the rate of exp gain but allow players of all levels to take part in other large scale combat without reaching 99. Since you server wide co-operation at all levels, there is no need to rush a player to level 99 for them to enjoy Vanadiel with their higher level friends. :3
It will also create a demand for low level armor again, and make synthing worthwhile again at lower levels.