If we ignore the Team War monsters/VIPs/Nemeses/Warmasters, and just use the regular F2P monsters in rough chronological order, then we can group them in generations of 8 monsters each. This is what I came up with for Dark:
DARK GENERATION 1
• Nebotus
• Darkzhul
• Lostyghost
• Sir Slumberly
• Dr. Hazard
• MMOnster
• Master Skeel
• Noctum
DARK GENERATION 2
• Barbatos
• Akhenotep
• Count Vlad
• Lilysha
• Klectus
• Krampus
• The Ringer
• Will Razor Face
DARK GENERATION 3
• Lotan
• Mommy
• Fenrir
• Erebus
• Jabaline
• Palhax
• Glitch
• Harusami
DARK GENERATION 4
• Ra’Zhul
• Scryb
• Hayman
• Fampira
• Little Red Furry Cap
• Baltasar
• Himass
• Hirondeleor
DARK GENERATION 5
• Jasastur
• Krugbo
• Faugnar
• Undertaker
• Keithor
• Montauk Creature
• Muerte McBlood
• Carlo Canbino
DARK GENERATION 6
• Legion
• Ixofex
• Dusk Aura
• Countess Flawless
• Warspellz
• Soul Hugger
Of course, this is not scientific, but groups of 8 monsters in each Gen would be sufficient to collect I think (like the spread of each library book page).
My aim with this is to spice up the meta... for example look at Dark Gen 5. Usually we would all use Jasastur as our denier, but in that group 7 of the 8 monsters are immune to possession! So actually Montauk Creature, with her AOE stun, becomes OP in that group.