I've been kicking around the idea of a viable Infinity +1 type for a while.
Null Type
Basic idea is that it's a type that "transcends" or nullifies the effects of other types. Pokemon that are the Null type tend to be those that come from places foreign to the regular Pokemon world.
Resists every other type. However, damage taken is 75%, not 50%.
When combined with other types, the Null type doesn't decrease damage taken from not very effective attacks, but will increase total damage taken from super effective attacks to 150% of normal damage. So, if Xurkitree is a Null/Electric type, it will always take 75% damage from a Flying attack and 150% damage from a Ground attack.
If a Pokemon has the Null type, then all super-effective damage it does is reduced to 150% effectiveness. Not very effective damage done remains at 50% effectiveness.
Most Null type attacks revolve around resetting the battlefield to a previous state or nullifying an opponent's advantages, like taking away stat changes, weather, or ability effects. The few Null type damaging moves have normal effectiveness against all other types except the Null type, against which it will deal 200% super effective damage.
Arceus - Null
Dialga - Null / Steel
Palkia - Null / Water
Giratina - Null / Dark
Porygon line - All Null / Electric
Unown - Null
Deoxys - Null / Psychic
Type: Null - Normal
Sivally - Null / (Whatever Memory it holds; when not holding a Memory its secondary type is Normal)
Cosmo, Cosmoem - Null
Solgaleo - Null / Fire
Lunala - Null / Ghost
Nihilego - Null / Poison
Buzzwole - Null / Fighting
Phermosa - Null / Fairy
Xurkitree - Null / Electric
Celesteela - Null / Steel
Kartana - Null / Grass
Guzzlord - Null / Dragon
Necrozma - Null / Psychic
