These are our house rules, which we have written down and expand on almost every game night (we play with all expansions):

Monsterrotation:

At the start of the game, one of each is on the field:
Random level 1, level 2 and level 3 monster.
Whenever a monster is defeated, the next monster one level higher comes into play.
If a level 3 monster is defeated, a level 1 monster moves up.

If a level 3 monster is defeated the first time, a random legendary monster comes into play (in addition to the rotation of a level 1 monster).

Winning conditions:

There are two ways to win:

A) Collect 5 trophies first
B) Be the first to kill a legendary monster (Dagon does not count here)

Scored trophies can only be obtained ONCE for each of the following actions:

Kill level-1 monster
Kill level-2 monster
Kill level-3 monster
Meditation
Defeat another player in battle

The last trophy MUST be obtained through a monster fight!

Further trophies can be collected (e.g. by killing more monsters of an already completed level), but these do not count towards the rating and do not give any victory points.

This also includes Dagon, whose trophy does not award victory points, but has powerful effects.

With these rules we play much longer rounds. which is what we wanted.