It doesnt NEED a new zone. It doesnt NEED new mechanics. It doesn't NEED new mobs. It doesn't NEED drops (apparently). Just drop Sortie Fomor in Dynamis upgrade the normal dienamis statues they spawn from by 10 levels and powerup Angra manu and the dragons. Donezo. You got Dyna-Beau [d]
Do you keep death on angra mainyu even though it'll (presumably) have 30mil hp to keep pace with other dyna bosses? That's not very fun design. You better set aside an extra 10 min for the chainspell death wipe, and bring a few tanks because it's going to kill them every minute or 2 even with best resist death available. Can't wait for the player complaints about how instant death mechanics are ***.
Do you reduce the HP so it feels more like an old Angra Mainyu zerg? Surely you have to keep the repeat WS nerf so it doesn't turn into a savage blade spam-fest(because everyone hates those). But, now you need a dozen different WS to be able to zerg it before the wipes. Hope your group has sufficient variety. Everyone absolutely loves forced variety.
If you're splitting it into 3 waves, how do you avoid creating the same running problem everyone hates about Sortie? Are 8 elemental circles enough for wave 3, or should you have more since it's a huge/harder zone? If you're not splitting or using those, what's stopping people from just going straight to Angra by leadening statues and killing the boss then leaving and never coming back? In any case, you'll need to place the mobs thoughtfully because a straight copy/paste would result in far too many mobs across far too wide an area.
You said no drops, does that also mean the mobs on the way to Angra don't drop anything? In that case, why do the content at all? You buy one Angra win and then never return. That adds nothing to the game. It's 18 man, so there'll be no difficulty buying it.
If the mobs on the way do drop heroism/astrals/cards/medals like previous dynamis[D], you have to be careful about how many mobs you add. Previous Dyna-D had special eye mechanics to prevent them being too easy to cleave, so you need one of those. If you are sticking with 1 wave of all fomor, you're creating the opportunity for groups to go in and get 3000 astrals in one smallman run. ***, that just removed another part of the economy because nobody will care to do old dyna-D any more, and it removes the ability for returning players to make gil in Omen.
Which zone cluster holds this new instance? Is there an instance cluster that can already provide it, or do you need to get a new server blade or requisition more server time? How is the server infrastructure split up; can one new physical server handle this for all 16 servers or does each individual server need to be altered? Is there already a streamlined way to connect the world server to the instance server for queueing, or is each prior queueing system implemented seperately and this'll also need it's own implementation?
This is far from exhaustive, and it's still primarily just looking at design decisions.. not potential challenges associated with implementing them. Maybe some of these things don't matter to you, maybe they don't even matter to the players. But, SE is doubtlessly going to consider all of them before releasing anything like this.
If I had the sum of my existing knowledge and the knowledge gained by working with their source code for 10 years, I suspect I could come up with a reasonable implementation for just Beaucedine and a roadmap for the rest in a month or two. But, they don't have even one dedicated staff member who can code.
They can't even start designing large scale content until they know they'll be able to get someone to implement it for them. It's not just a matter of 'finding a coder' either, they need someone who has worked on XI in the past and understands a good bit about it or the amount of work increases exponentially. There are likely only a few people who fit that criteria, and being able to borrow them relies on more profitable parts of CBU3 being able to spare them. So, while something like this is certainly doable over a longer scale, it's not entirely within their control and it's certainly not a quick copy-paste.