For horizontal content, I hope that OTM makes more zones to level in. OaC has a bunch of zones that have mobs with similar levels, giving people a variety of places to level up in. In CH there are a limited amount of spaces, especially at the higher levels. For instance, if I want to level in a Firbolg cave and someone is already there, I can't level up because there is no where else to do it. Same with Mordris's cave. I just have to log out and find another place to go. When I'm logged out OTM isn't making money off me.
Now Gelebron's tower will help this somewhat because we have learned that level 170 is the ideal level there, because that was a prerequisite to get into beta testing. So there will be some overlap with Carrowmore to level in. But as the level cap goes up (190+) the only place to level will be in the tower. Which will again crowd everyone into an area making them fight over mobs.
I hope that new areas will be added, but have the same level of mobs. And please learn from the mistakes in the sewers, and the successes in Carrowmore. There needs to areas to level up in with mobs tightly clustered together. The sewers doesn't have this, and sucks to level in. Therefore, no one goes there except for bosses and hawkhurst chests.
Personally i did lvl a bit in the sewers, on the zombies and the lizards but i do agree with everything you said. I am no endgame player myself but i think i understand how annoying this all is. When the "underworld"(u4) came out it gave a lot of new places to lvl, even if there were already mobs of those lvls. Something similar should come after the currently planned updates, staying at the cap set by the tower.
This will allow for more areas to lvl and maybe more quests as well. However, there have been some "mistakes" in fingals and sewers, for example the huge spaces in the entire western half of fingals and the kinda deserted southern half of the sewers.
Just my opinion, like i said, im no endgame player myself