New mechanic: "Hidden type" (HT)

Give an extra STAB to the pokémon when he uses a move of the same type of its hidden type

However, Hidden types don't give an extra weakness or multiply the current weaknesses

Ex:

Gyarados has "dragon characteristics", so it has dragon as hidden type, therefore its dragon moves get extra damage

Other suggestions to the First Gen:

Venusaur - ground
Charizard - dragon
Blastoise - steel
Butterfree - psychic
Beedril - flying
Pidgeot - steel
Raticate - poison
Fearow - dark
Arbok - dark
Raichu - fairy
Sandslash - rock
Nidoqueen - fighting
Nidoking - fighting
Clefable - psychic
Ninetales - ghost
Wigglytuff - Fighting
Crobat - dark
Vileplume - fairy
Parasect - poison
Venomoth - flying
Dugtrio - rock
Persian - dark
Golduck - psychic
Primeape - Normal
Arcanine - dragon
Poliwrath - ground
Alakazam - fairy
Machamp - steel
Victreebel - ground
Tentacruel - dark
Golem - fighting
Rapidash - electric
Slowbro - ice
Magneton - flying
Dewgong - fairy
Muk - ground
Cloyster - steel
Gengar - psychic
Steelix - rock
Hypno - fighting
Kingler - bug
Electrode - steel
Exeggutor - dragon
Marowak - ghost
Hitmonlee - normal
Hitmonchan - normal
Lickitung - dragon
Weezing - dark
Rhyperior - steel
Blissey - fairy
Tangela - poison
Kangaskhan - ground
Kingdra - ice
Seaking - dragon
Starmie - steel
Mr. mime - normal
Scizor - flying
Jynx - fairy
Electivire - Fighting
Magmortar - fighting
Tauros - ground
Gyarados - dragon
Lapras - fairy
Ditto - psychic
Vaporeon - normal
Jolteon - Normal
Flareon - normal
Porygon - psychic
Omanyte - bug
Kabutops - bug
Aerodactyl - dragon
Snorlax - fairy
Articuno - none
Zapdos - none
Moltres - none
Dragonite - water
Mewtwo - none
Mew - none