My thoughts: you could...
[1] Zone Difficulty: Make Vex Thal be Tier 9 in the end, but also span multiple tiers. I always enjoyed the pseudo-tier style of DSK1 and DSK2. Vex Thal could borrow this idea and span multiple tiers. Perhaps 7-9 or just 8-9. The flags could then be compatible with other zones. That is the concept, and here it is in practice:
A player can be flagged for DSK1, and then that flag can grant access to VT1.
DSK2 will grant access to VT2 (and perhaps vice versa?).
HoH will grant access to VT3.
A Vistrinda kill will grant access to VT4.
Chaos kill => VT5.
Khasva kill => VT6.
You said 16 events in VT, right? Now, I should admit that I never raided VT on EQLive. I don't know if it can be split in 6 parts, and if that even jives with the plot, but hey we're here for THF and not EQLive, so we don't need to 'emulate' EQLive perfectly, right?
But Noren, why would we ever want to do this?
Two main reasons:
[1.1] To avoid stagnation across a larger group of the THF population, and not just limited to the end-game people. If people are floundering in DSK2, burned out or at wit's end, for whatever reason... perhaps a side-step to new content of equal difficulty could keep things fresh and leads into my next point,
[1.2] To provide complementary content to aid guilds who need more content at their difficulty level. One guild who can make good progress in Nadox but hasn't made any headway into HoH bosses might benefit from a few more bosses to kill each night at their difficulty level. Whether it be to pass a little more time on THF or to get a few more upgrades each night to give them an edge against the next boss - complementary content has its uses.
[2] AAs: A new take on AAs and raid boss kills. I was thinking that it could be neat to have a fairy spawn upon the death of each VT boss like they do with DSK3 bosses, but offer something different. Instead (or in addition to) 3 flags, they could offer an AA to 3 players. This AA could be something as simple as +Spell Dmg or +Dmg Bonus for physical hits (is that possible?), or it could be an empty shell AA where acquiring 15 ranks (thus 15 boss hails) unlocks access to an expensive 500 AA ability that we can spend our stockpiled AAs on.
[3] To build on Rude's idea of an item that grows in power based on the boss kill's the player is flagged for: have an item that can be obtained at the zone-in, from an NPC, that grows in power with each boss flag. This item could be exchangeable like with Sheever in the forest zone, where you can hand it in to cycle through different rewards. This could turn a 1hs into a 1hb into a Piercer into a 2hs/2hb into a caster staff, at the risk of losing the augments in it. An evolved or higher rank weapon will trade for that same rank. Etc etc. It doesn't have to be weapons either - that's just an example.
[3.1] I would like to see another class spell like the Death March I-IV line introduced in this tier, too.
[3.2] Also, an item like the Queen's charm would be handy with a clicky that teleports the player to VT. Perhaps that could be the key itself. And then the key could be upgraded to reduce casting time and stats if there are any.
[4] Boss idea: I would like to see a boss that goes into timed phases where damage immunity changes. The fight starts out with the boss vulnerable to all types of dmg. Then a phase kicks in where the boss is vulnerable to only poison dmg. A couple things can happen in this situation. The boss can take increased dmg to poison (x2+ modifier?), or perhaps normal, but then things can get tricky where the wrong type of damage will heal the boss. This fight will emphasize control and damage diversity. And the phases being time-based means that anyone could technically do it, even if they lack poison type damage. They would simply wait until the next vulnerability.
[5] Boss idea: The spell damage boss with no hate list. This boss will be untankable. The idea is that the boss will rotate from player to player on the hatelist and either attack each random player for a fixed duration or until a fixed amount of damage (200k) is dealt. The boss would teleport to a random location (to keep the melee from getting lazy) and nuke/dot a raid member while the healers would simply have to switch targets quickly and keep up. No one would be insta-killed, there would be time, but this would be a fun bit of a chaos and a way to say 'neener neener' to the many uber tanks out there on THF who are just average joes during this fight.
[6] Boss idea: The contagious boss. This will infect X players on the hate list with X different spells. The spells could be 30 seconds in duration, and have the ability to spread to nearby players every tic. The spells can be a mix of beneficial and detrimental spells. One could be a dot that saps away 25% of your max health every tic (meaning the full duration with no heals means certain death). Another could be physical dmg vulnerability (don't infect the tank!). Another could be healing weakness, another dmg output weakness. In addition, there could be beneficial spells that you DO want to infect others with, or perhaps each weakness could be paired with a strength (i.e. healing weakness and dmg dealing strength).
[7] Boss idea: The walking bomb. This boss could spawn 1+ adds that are walking bombs. They could be fixated on one target and will do an AOE direct dmg should they reach their target. Here is where things can get trickier. They could be on a simple 30 second timer, and you just need to keep them away for 30 seconds, or perhaps they need to be killed before they can reach their target. It could be fun/mean to have their target be snared or limited to no movement modifier with a debuff, causing the target to have no choice but flee in a straight line or risk peril. The size/shape of the room could dictate how far they can run and if the raid's assistance is required for them to survive.
[8] Mini-boss idea: Eye of Zomm reverse Pac-man. Everyone in the raid can hail an NPC for a no rent item. This item spawns a special Eye of Zomm. When the event starts, everyone is teleported and rooted to the center of a maze. Gear is no object in this event. Clicking their no rent item, they will spawn an Eye of Zomm that has 5 minutes of life. When the event starts, all players are silenced so that no more Eyes may be spawned. The goal? Survive. There will be one or two NPCs that act like Pac-man. The Eyes are the ghosts. Since you see out of your Eye of Zomm, you have to outsmart and outrun the slightly-faster-than-you Pac-manish NPCs. So long as one Eye survives, the raid wins this mini-boss encounter and a treasure chest is spawned. Perhaps the loot table could be altered by the # of surviving eyes. This is a very rough idea, of course. We know pathing is very challenging to perfect, so maybe a maze is unfeasible with the potential for mobs to run through walls or fall through the world. The duration could be 90 seconds, or more, or less.
[9] Mini-boss idea: Mutiny! This mini-boss charms a random half the raid and sends it after the other half. It is the duty of the other half to defeat their charmed brothers in arms. If the AI is too clumsy to handle raid-geared players appropriately, then maybe an unbalanced portion may be necessary (70% vs 30%) to ensure a good show!
[10] Boss idea: Aid the NPC! Your raid must assist a powerful NPC in defeating the even-more-powerful boss. Your NPC ally will be tanking and dealing all of the damage to the boss. It is your job to assist him. Keep him alive with heals and buffs, but that's not all... waves of adds will spawn throughout the fight to keep the raid busy. Don't fall behind and don't let your ally fall.
[11] Boss idea: Drain/gain zones. This fight could involve three zones. On one end is zone A. This is where the boss resides. In the middle is zone B. And out of spell range of the boss is zone C. Throughout the fight, zones A and B will change randomly to drain mana or HP+Endurance. Zone C will always be a rejuvination zone, but you will be out of range of the boss/tank. Should mana/hp/endurance fall to zero, that player will die (hp does this naturally
), so zone shuffling will be important, and tank switching inevitable. Healers cannot heal the tank from the rejuvination zone, but they could still reach zone B residents. With heals, players could tough it out in an HP drain zone, but they'll have to keep an eye on that endurance.
Obviously requiring much less kills to evolve, but still... Sort of an added benefit to killing a mini or boss as opposed to killing trashmobx 20 trillion times. Makes killing a named that drops nothing you want still useful for everyone. Would also be awesome if there was no real reachable "end" to a new set of evolve items 
