1. Griffonhold - 33, Castle Level 27
3. I see absolutely no reason to NOT merge realms.
4. Choosing a realm that you merge with would be great. The King SHOULD NOT be able to move several players to the same realm, as there are people on current realms that you would not want to play with if possible. There should be a menu that pops up when you log back in that allows you to choose from a list of A VERY FEW amount of servers. Right now you have 90. That's like 80 too many.
5 - There are many concerns. I'll cover all concerns below.
Merging is absolutely without a doubt needed. As others have said, the primary concern is how to give new players a fighting chance. I'll address that first.
Have a reasonable might comparison for fights. You already have a rule that you can't attack someone much higher than you (I believe). Well, you need to put something in place that disallows very high players being able to attack very low players. For instance, someone with 300k power should not be able to attack someone with less than 100k power. The actual power differential will be your own determination, but if you have a server with 300k power+ players, a 100k player isn't going to pose any significant threat, so being protected should be fine. As a bonus, this will actually DISCOURAGE pay to win players from buying so many gold troop stacks that they have so much power that they can't really do anything. As an example, say you had 400k power and you wern't allowed to attack anyone lower than 75% of your power. So at 400k power, you could only attack someone at 300k or above. So if you have an insanely high power rating due to spending tons of money, you're actually taking away from your ability to play the game, but at the same time, there is a fairness that is maintained in the game. Troop buying should be for replacing troops quickly, rather than amassing so many troops that you steamroll anyone.
Server should be done on a choice basis PER PLAYER. Trust me when I say many of us are tired of playing with some of the people on our servers. You have backstabbing little bitches like "Magic" on our server that it would be amazing to get away from. Also, with migrating being FREE with absolutely no downside, my realm, and likely the same issue on other realms, have congregated into two kingdoms while effectively allowing the others to die out. So many of us aren't in a kingdom BY CHOICE, but rather because you have to be in order to survive.
When it comes to cities, I would recommend just resetting the entire world. Some may thing that this would suck....but in practice, I don't think it would. True, it would be a MAJOR dogfight to be the first one to the biggest cities, but with the mercenary troops (or whatever) in the cities initially, it's not like it would be a race that is solved in the first 15 minutes of server re-opening. Cities will need to be cleared, battles will need to be fought, and the cities would need to be earned, but this would also naturally reset the boundaries that have been so messed up with migration and server death.
As I've said before, you have 90 servers, which are probably about 80 servers too many. But, I realize that you have THOUSANDS of registered players, so how do you fit them all on 10 servers? Your solution is simple. First of all, you can probably delete about half of the players that originally started the game as inactive. Then delete the other half of them because they have multiple accounts. That leaves you with about 25% of your base being actually active players. You have 10 (or however many you realistically need) PLAYABLE servers and some "Ghost Servers" that are unplayable. When a character goes inactive for say, 2 weeks, they automatically get put into a Ghost Server, until they log into the game again, at which time they will choose which server they want to play in. This will maintain that ALL players actually in your playable servers are active at least within 2 weeks. From there, you can control your actual population MUCH easier.
Make Migration available ACROSS REALMS and not just across kingdoms. DON'T MAKE EITHER FREE. Charge something reasonable, like 250 linari for a kingdom migration and 500 linari for a realm migration. You guys gift enough linari that anyone would see those prices as reasonable. With that migration, EVERYTHING carries over. The only issue you will have to solve with that is whether you allow people to migrate into a kingdom on a realm that has the most cities. That'll be up to you.
I've seen requests for bigger worlds. I would recommend against this, UNLESS you speed up the damn carts. I can't imagine having to escort a cart for 15 minutes to go across the map. It's ludicrous. Honestly, you should really at least double the speed regardless of what you do. Or at a minimum, make only escorted carts move faster. Think of it like using your mount to pull the cart.
Implementation of a server merge would be that EVERYONE has to choose what realm they want to go to right after the update. So after they log in, they choose right then and there. HOWEVER, you will NEED to announce how many servers you are opening beforehand so those that want to continue playing together can choose the realm they want to be in BEFORE the update. Also, allow a "random" button for those people that have a panic attack when making decisions. As long as you implement realm migration as well, they can always change their decision later if there's a major asshat on the server that everyone hates. (*cough* Magic *cough*)
That should be enough to go off of for now. Cheers!