Every idea you outlined either wouldn't work on a private server, or is too vague to even criticize or tell if it would be good or not. I'll prefix by saying that you can make an argument that many of my points are invalid if you're not looking to make a "successful" server, but unless you want an empty server to mess around on, you can't just choose to ignore what the majority wants.
Have fun advertising a product where at least 90% of your target audience is sick of Flyff's boring butt grind. This is literally most people's #1 priority when looking at private servers. You might as well not waste your time if you even think this would be successful.
Again you're ignoring your audience. No one ever played Flyff for it's story. 100% of your target demographic played Flyff and liked it. It's very unlikely that they wouldn't enjoy a server without a story. That being said, why not focus on improving some of the aspects that define Flyff? Like its fast paced PvP or the quirky graphics. On top of that, it's fairly challenging to make a compelling story for a game that will actually hook in today's gamers. If most AAA games can't do it, what makes you think you can when said AAA games can hire the best writers in the industry?
Well it's easy to argue that Flyff's party system is poop. It actively discourages leveling with more than 2 people in one area. However, this is extremely vague and I really don't think this is a key idea. It's more of a small detail that most players probably won't even pick up on.
There are plenty of party dungeons already. The issue is that they're Ducking mindless. Due to a poorly made combat system, dungeons are either way too hard for the group or way too easy. If they're too hard(always based on gear or class limitations), its usually impossible for the group to beat the dungeon without getting better gear. If the group has good enough gear, it's usually extremely mundane, where the damage dealer can afk with a ringmaster spamming heal rain or something.
In most successful games, dungeons have challenging mechanics that actually require precise input by the player, rather than just a test of whether or not your party's stats are good enough.
New game content/New armours/New endgame/New guild system
If you put as much effort into the implementation of these ideas as you put into coming up with them, you'd barely have a vanilla v15 server running. How about elaborating on what you'd actually add. Any idiot can tell that Flyff has no endgame content. Have any ideas on what would work as endgame content for Flyff private servers? Because based on this post, anyone that's reached the max level on officials has as much insight into what Flyff needs as you do.
I'll end by saying Flyff in general is a fairly poorly made game. There isn't much actual gameplay and most of the players are only still playing it for nostalgia. I've seen the concept of a "new world", but even if you implemented it flawlessly, it still wouldn't be very effective since a huge majority of your players just want to play an improved version of Flyff without shoehorning incompatible elements of other games into it.