New monsters are up on the wiki.
Soulhugger counters Necromancer hard
Kozorg, Frosilka are both strong counters to Thallassa unless she's high enough to disable their trait and freeze them (then you still have 2 other monsters) Depending on what book Eeltron goes in, he's also obviously trouble for her.
Warmaster days are already numbered with the new monsters they are releasing. Looks like it takes about a year to make the newest stuff obsolete. Not long after last Nemesis was released warmasters showed up. Now with 1 warmaster to go they are releasing monsters to counter them. Can't remember how long between last VIP release and first Nemesis but it wasn't that long either.