Close the AH 3days for new Auctions prior Merge, so every item not sold Merges
History?
Reset Islands.
But, to make it easier, im sure its gonna be a 2 Step Transfer.
First they Merge several Server which are close to empty on EUC and EUW, like EUW is left with Moonkeep and Punika. And X + Y on EUC.
2nd Step Replace X+Y with Moonkeep and Punika.
Sofar, im sure the biggest problem i think, is Character Names, EUC its already Problem to find a Name. Now having to probaly rename any Character from EuW, gonna be a Disaster.
The only hope is they working already on a System like Charname_Server. That system would help even without a Merge at all, One Name for the Whole Region was not a good idea from the beginning, and someone did not thought much about it years ago.
Option B , but i think it would be a Bad Option. Connect EuW Market and LFG.
Ping differences, even databases talking to each other. Never gonna see that option happen.
Option C, not a Great option, atleast aslong they not solve the Name Problem.
You can put anything, really anything, in a Universal Storage, mounts, pets, Customs, Hone items, gold silver. Pretty much anything. Which u can empty on a Server of your Choice.
One single time.
South Vern Pass for every Character you own.
Some extra ideas:
They could create additional Tickets you can Claim on the “old” region, like one for each 50 mokoko you collected. For every Isle soul u Collected. etc
And if you use these Tickets, you get “random” completions. After all its still a RNG Game 
Overall, Option C is the Worst, and can lead to many Complains from other Regions.
I only know about KR, but whatever, they had never to work with that amounts of Servers, and they just tried out if they can make some quick Cash with selling Transfers, and failed.
Now its a whole new Situation, and they have to put some Real work on it.
Losing whole Regions gonna hurt, EUW is not the only one, just the worst right now i think.