I came up with a new variant and play tested it along with a mid-boss variant and the Split Stone and Grounded Boss variants. Check it out:
Factions Variant – Heroes have banded together in factions to find the stone for the common cause of their combined peoples. Requires four to eight players. Each chooses a hero or some may choose two if there are less than eight players. Players should not choose both of the heroes for their faction one right after the other. Players then divide up into factions. A faction may not consist of two heroes of the same training (e.g. a faction may not have two Talent heroes).
The faction moves together as one unit. Both heroes in a faction still get two actions, but movement costs both an action since they don't leave each other's side. If one hero is out of actions the other hero in the faction may not use any of his/her actions to move. Abilities that grant characters extra movement (e.g. Paelleal's Blaze Step) may still be used to move the faction without costing the other hero any actions.
Loot is collectively held between the members of the faction except in the case that one hero has a particular treasure equipped. Encounter cards are also collectively owned and the advance action applies to both heroes, costing both an action. Abilities that make a hero able to advance easier than normal (e.g. Dorgan's Xenophobe) may be used, but only that hero may advance, only that hero is charged an action and the relevant encounter card is discarded and may not be used to help the other hero advance. Only one hero in a faction may use the summoner stone.
Heroes in a faction together may not sabotage each other. No hero in a faction may assist any hero in a different faction. A hero may not take two battle actions in a row; their comrade must attack before taking a second battle action (exception for abilities like Vargagg's Fury). Add half of the life value rounded down to the life value of monsters and half of the attack rating rounded down to the attack rating. Add one to successes needed for traps. Heroes combine appropriate skill rolls to disarm trap together. If the trap goes off then both take the damage. When a faction stumbles upon a monster and gets a free battle action only one hero may attack and the other may assist.
If one of the heroes in a faction is knocked unconscious his/her partner may not leave the tile their partner is on. If one of the heroes in a faction dies then the partner receives all treasures and artifacts from his/her fallen comrade and may continue the adventure solo.
Factions Variant 1.1 – Like the above, but when one hero dies the comrade also becomes imbued with the spirit of their fallen comrade and enraged. Receiving the help of the spirit of their comrade gives the following advantages: 1) they receive a +1 bonus to brawn, magic AND skill; 2) they get to add half of their total life rounded down to their life value as a new life value; 3) they get to make one of the learned abilities of the fallen comrade their own, but not the summoner ability. In their enraged state they may not escape any monster or hero and they may not assist any other heroes. Also, they must fight all monsters or heroes they share a tile with if they have the actions available.
Comments: It's actually pretty balanced and offers players new ways of thinking strategically about who they ally with at the beginning of the game and how they proceed through the dungeon. I felt like there was a good combination of cooperation and competition. Feel free to ask for clarification to the variant rules or about the run through.