The great difference between a game like Left4Dead (And the like: L4D2, D4B, CoD Zombie mode…) and tide game for me is the focus on the melee and short ranged combat (favouring being as close as possible to the enemy)
Requirement:
- Possibility of having both melee and ranged weapons
- Enemy roster with tiered units
- Special enemies (Lord, Monster, Elite and Specials)
- Varied places
- Varied Character potential
- Low Fantasy (minimum)
Eg:
Warhammer Fantasy (And AoS):
- Dreadfleet: Vampire Coast Alliance (VCount, Ghost Pirate, Tomb King, Undead Skaven and Chaos Dwarfs) vs Grand Alliance (Empire/Sartosa, Manaan/Sartosa, Dwarf/Barak Varr, Elf/High Elf Navy, Sartosa/Araby)
- Greenskin/Destruction: Greenskin (Goblin, Orks, Troll and possibly Ogre) vs Order aligned faction (Dwarf, Human, Elf baring DE, LM)
- Undeath: Undead horde against the livings
- Chaos: In part contained in VT2
Warhammer 40k:
- Tyranid: A game featuring any faction baring Chaos, so Empire and Xenos Alliance against a Tyranid incursion
Other Modern:
- Call of Cthulhu/Delta Green: Normal Humans against Elder Gods follower and other cryptic beings
- Dresden Files: Battle ground Spoiler Chicago Alliance against the Fomor
- Weird War 2: Set in a parallel WW2 setting where most (if not all) of the Nazi experiment worked/succeeded (So Zombies, Vampire, Werewolf, Augmented beings…)
- SCP: Working as either SCP agents or as Chaos Insurgency fighting to contain/liberate Anomalies