I also think that creating events with certain levels of difficulty would be paramount...
I know that the majority of the server population is typically T5-T9 (myself included) which makes it easy to overlook (or pretty much forget about) those who aren't in a high tiered raid guild or are solo players. Those muddling through and trying hard to complete that next quest or seeking that new major upgrade... Many times these people can get mired in a frustrating place and since much of the post-70 game is guild oriented, this can become a major frustration -- even a game breaker for some people.
Events centered around ideas that allow equal opportunity for noobs, lowbies, solo-players and uber T9 raiders alike would be cool.
Also, combined events, where the GM's pair high and low level toons together (preferably at random instead of allowing pre-chosen groups of pre-existing guildmates to pair up) to achieve a series of objectives through cooperative play and strategic efforts -- i.e. Have a level 70 group with someone =>55 to go on a scavenger hunt together. Place a heavy hitting mob with a huge aggro radius in front of a small tunnel with a ground spawn or NPC in it. Make it so the 70 toon has to engage and kill the boss while the lower level has to retrieve the ground spawn (or hold a dialogue with an NPC for the next clue) as the higher level holds aggro (otherwise it AoE's and kills the noob toon thusly depopping the ground spawn or NPC, equaling a fail). Make it so the trigger for the ground spawn/NPC is engaging the mob and then make it so the mob has a fanatic morale and won't allow the 70 to leave without killing it (i.e. have it summon the 70 back if they try to leave). If either dies the event is failed. The event is completed when all objectives are achieved by both toons in the series and both are rewarded with items that enable furthered growth for both toons (separate level specific rewards).
Another similar idea is to have events where the 70 toon has to fight an invulnerable boss or series of bosses who use an irresistible root spell on the 70 while the =>55 runs around collecting ground spawns that contain clickable items - some of which will drop the boss mobs defenses, have other detrimental effects, or possibly even buff or heal the boss mob(s). Both will need to learn what does what and work together to complete the battle requiring teamwork and communication. Have rewards like event specific augs that are slot 4/8/10/16/30 with decent skill mods that are comparable to augs you find in T1-T3 for the lowies (10-35AC 125-175HP/Mana/End +1 stat caps and +12% skill mods [varied]) and something close to the Aviak/Shark/Kodiak augs for the 70's (25-50AC 175-250HP/Mana/End +3 stat caps to random stats and +20% skill mods). This way it is not the MOST powerful stuff available but is still nice enough, and well enough worth it to do a fun little thing that encourages comradery that bridges the level gap.
I also think that it would be cool to have a zone, or zones, where level 70 characters can team up =>55 characters (you could easily utilize Legacy of Ykesha, Depths of Darkhollow, and Prophecy of Ro zones since most of those don't exist in the current game -- I guess this is more an expansion idea than a GM event though) and perform missions together like a mentoring system. Have a quest giving NPC that generates a random quest in the zone or zones (kil x# of this mob, defeat that boss, collect these drops, deliver this item to X -- etc) and have two turn in NPCs, one that only speaks to level 70 toons and one that will only speak with =>55 toons. Grant a double exp bonus to the lower level toons with slightly OP gear (like SH/T1 quality but non-level specific -- of course no drop/lore) and maybe offer rewards like dyes, a Lillu's distiller, and other rare but commonly useful items through the incorporation of a points system (like your Vale LDON type vendors) that would motivate level 70's to want to seek out lowbies to help. Of course limit the number of runs to like 3 a day so it cannot be overly exploited by any one toon. Maybe even throw in a few items like a tank, melee, healer, and caster type weapons for the level 70's that are near to the quality of a donor type weapon if they participate 300 times or something...
I think things like this would benefit the community since it allows the higher levels to develop friendships with lower levels -- which in turn gives the lower levels the ability to get information and experience mentored to them from experienced players and opens the doorway to guild opportunities through friendships.
You could also make timed events that center around trade skills.
"Lum the Mad Goblin Baker" needs a Baker's dozen of Batwing Crunchies and a hot Gnome Pot Pie for the Ogre Princess Lumpo's wedding and he cannot fill the catering order fast enough! You must go into [whatever zone] and get the items needed from [recipe list] and use Lum's oven [located here] to fill the order. Make it so that Lum's oven randomly generates successful and failed results (returning undercooked, acceptable, perfect, overdone, and burnt to a crisp cooking results). Have different dialogues for the turn ins and grant +1, +5, and +10 baking skill rewards for each completion. In the end, depending upon points earned, you can also get prizes based on the total number of completions within the time limit.
General Hayward of the Freeport Militia is planning a raid Grum Bowleg's forces in Oggok. He needs you to make 20 steel longswords, 30 Kite Shields, or 50 Fine Plate Breastplates for the invasion. Have a load of ground spawn components that can be easily found and looted to fulfill the required amounts needed and have special, event only recipes and forges to perform the combines in that result in non-player wearable (or uselessly low grade) weapons and armor for the turn ins. Make it a timed event and have the reward be based upon the choice of 20 (easy), 30 (challenging), or 50 (difficult) items to be made. As with the baking idea, the forges will return random results and luck will play a large factor in success. Allow the combines to advance your smithing as also with the baking idea.
You can twist up these with funny and entertaining ideas with brewing, tailoring, fletching/bowyering, etc. Like for instance, Hairytoe the Troll is in love with Gwendolyn the High Elf but she won't give him the time of day. Hairytoe needs you to make him a love potion (or at least something to knock her out a little while) so he can profess his love for her (or steal her away in the middle of the night -- whichever works).
The ideas flow like wine...