Just like the 10v10 and 100v100 seasons are stuck in Groundhog Day mode, Lands of Mist are stuck with the same monsters they've had for a couple years now. The monsters in LoM were changed at least once in the past.

I recommend expanding each the lists for the average levels and guild levels, instead of only four or five monsters, have ten. From the expanded list, run an automatic script at the beginning of each month to pick four or five monsters to allow guilds to fight that month. The script could cycle through all monsters, removing one or two each month, scrap the entire list and pick new monsters, or it could remove one and pick a random replacement.

Fighting the same monsters every day is getting old...

It would actually help more if new monsters were added to the game, with some of them going into LoM, but I wouldn't want to push too hard for things the game needs to survive...