Im designing a tactical rpg/management game. The dungeons work similarily to those in "the darkest dungeon": You enter a randomized dungeon and will try to "complete" it (get the completion reward) without loosing too many heroes.

I would like the completion reward to actually make sense. In darkest dungeon you just "got" some loot once you completed the objectives (such as exploring 80 % of the rooms). No explanation to where those items came from.

I have only two type of dungeon goals that makes the "completion reward" logical:
  - Kill the dungeon boss (in that room the boss has a phat treasure chest with the rewards)
  - Find the dungeon treasury key and treasury (once you find it the rewards are in that room) 

Other type of objectives such as "kill all enemies" or "explore all rooms": here it's less obvious why the player is "rewarded" with loot (gold, nice swords etc). Any idea on how to expand the working goals or fix the non-logical ones? I'd like to keep "finishing" the dungeon much more rewarding (since higher risk) than to just try out the dungeon and run away.

Some random ideas that might help.

A) Kill all monsters could be a sort of ritual. Collect something from each enemy, take to a particular room to unlock the reward. True, it's similar to find key to treasure, but forces the player to kill every, or nearly every monster. The one caveat here is to perhaps avoid excessive back tracking.

B) Lift a spell. The players must visit every room and perform a sort of dispelling spell. Once the spell is lifted, it would be possible to create a sort of portal or materialization for the reward

C) Traverse. Escort somebody through a dungeon. If you get to the other side, you get paid/rewarded.

D) Multilayer archeology. Players go digging at some excavation site. The idea is to have a small number of mini-bosses with locked pieces of dungeon behind them. It potentially increases the risk while still being logical.

E) "Timed dungeons". Players essentially don't get a final reward, but rather travel the dungeon 'horizontally' or 'vertically', increasing the challenge (and rewards), or staying at the same more or less. They must, however, leave before some event (say, 40 total rounds before the dungeon starts collapsing and they are 'evacuated').

Good ideas! A variation of escort could be save a npc: find the imprisoned npc (in one of the the dungeon rooms) -> escort him/her out of the dungeon -> get paid when you are safely back in town.

A goal such as "kill all enemies" could also be rewarded "back in town" by the town (that wants to get rid of those pesky giant spiders). It doenst have to be a chest with swords right when the dungeon is cleared. This becomes similar to a typical MMORPG "quest" though. Not sure that is good (but maybe you dont need to "pick up" the quest beforehand; you stumble upon a dungeon in the wild, clear it and are rewarded for your good service to the community later when you get to the town).

"Kill All Monsters" or "Explore All Rooms" could be more a post dungeon reward, where the party was contracted to do that (local village wanted the monsters cleared out, someone wanted a map, etc.). An employer could be behind many objectives.

"Escort" objectives give you lots of options. It may be trying to get through a tunnel like dungeon (e.g. through an otherwise impassable mountain that would take weeks/months to go around), or maybe an archaeologist or such wants to investigate something, so you need to get them there, hold the position while they do their thing, then get out.

Rescuing an NPC, or NPC party, or at least discovering what happened to them, lots of variations there, reward would either be from the NPC themselves or someone that sent you to investigate.

And of course, you can just have dungeons that almost immediately throw whatever the original objective was out the window. Maybe you went in to find out what happened to a local wizard, only to find they are raising an army, etc. Not dieing and the world not ending being the reward, but maybe people would be grateful and some good loot along the way.

Might look at the sort of things people do in Dungeons and Dragons etc.

I like this idea a lot. Could it be possible you could enter a dungeon and whatever prize you get what be to upgrade to things like mage or wizard status? Like gaining skills to be one.

You can have a random monster drop some loot, since the player doesn't know which monster drops loot, he may have to kill them all(or get lucky)

What if there were items hidden or randomly placed on monsters that contained experience or gold modifiers that only worked while you remain inside the dungeon, so you need to go around mugging/sensing monsters until you find the one carrying a 2x XP modifier and then you can fight the entire dungeon.

What if there were monsters that had to be lured into different areas that served as keys to unlock rooms?

What if there were invisible rewards hidden and the only way was to explore every square inch?

What about a mysterious NPC that has the ability to appear whenever he likes, that is interested in gathering some type of resource from dungeons, and uses what you gather to create powerful or useful items, for the purposes of some kind of cryptic research.

What about a weak weapon or spell that absorbs enemy energy when they are destroyed with it, that is used by an NPC to create new abilities or allies.

What about weapons that are given to you, and kills made with them are considered a tribute to different gods. You have to choose which god to pay tribute to, in exchange for powerful rewards like Meteor magic spells or a sword that has unlimited durability?

Just some ideas. They don't tie directly to exploring the dungeon itself, but I think you didn't like that idea.

You might wanna make a spreadsheet or what and balance out your loot in the game. This is what i normally do when making these types of game L)