This thread topic pops up every once in a while, so here's a quick FAQ about it:
Q: How about adding more quests or rewards in the mid-level content?
A: I can do that, but it takes away production resources from current end-game content developement, and I'm sure that nobody wants that. If I design new quests, items or encounters, it doesn't matter what level they are; the amount of work is mostly the same.
Q: Can't you just hire someone to do it for you?
A: I tried that in the past, but it never worked. Most people simply don't have the programming knowledge required to make content in Gaias. I can outsource stuff like item creation (Ihaz and Box did a great job on this and I'd be more than happy to have them work on new items in the future), writing dialogues for quests and stuff (Jumbo helped me on this recently), but actually making these completely without my input requires JASS knowledge. And everyone who does have sufficient knowledge of JASS either works on their own projects or just writes random systems without participating in larger projects. In a way, the hive community is really fucked-up in the regard.
Q: Do you have anything planned for the midlevel zones that won't take away precious production time?
A: Yes. With 1.2C, three new features will be added to the game: repeatable quests, rare mobs and treasure hunting.
Repeatable quests will reset every 30-60 minutes and will usually add an opportunity to farm xp and gold aside from mindless mob grinding. For example, repeatable quest goals could be to craft certain items, find monster drops, etc.
I'm experimenting with randomly generated repeatables aswell, but can't make any promises about that. One or two of these repeatables could be in the midlevel zone.
Raremobs will basicly add something new to do for solo players and explorers. There will randomly spawn some pre-made variations of strong non-boss units around the map, that drop various items normally only obtainable from bosses. Since these mobs will be rare, this will be designed in a way not to devaluate group-content in the game. You can still get your stuff much faster in a group. But it adds an alternative for those that like to play solo.
Treasure hunting will be my take on random lockboxes in the world. Just like raremobs, you can sometimes find a treasure box at hidden places across the map. To open them, you need to find a key first. These keys get dropped from the enemies in the zone.
The treasure boxes will drop materials and sometimes also boss-items, making them obtainable for solo players who like to do exploration.
Q: Why not put more stuff in the sewers?
A: Ahh, my favorite topic. I gotta say I liked the sewers a lot, initially. The optical design is great and it certainly adds a different flavour to the game in terms of interior design. Initially, the sewers were meant as a "real" dungeon. But it turned out that the playability was lacking. The corridors were too narrow for boss battles; the pathing was bad and the walkable tiles just didn't look good. So I dropped that and just put one of the main quests there to progress the story. I also added a side quest later by popular demand.
To be honest, I just don't feel that the sewers should have monsters. I liked the idea of making Gaias Retaliation Part II some kind of exploration quest that doesn't require killing. It adds some kind of horror element to the game. And horror works best when you just anticipate what could be there, even if there isn't anything in reality. The Shade encounter kind of plays on that element.