While thinking about weird mana (purple mana?), it was just a silly thought to tap untapped lands you control to pay for stuff instead of actual mana. However silly, this led to a new keyword action that can be used on not just lands, but other permanents as well.Disable
a permanent = tap an untapped permanent you control
"Purple" mana = disable a land
As an additional cost to cast ~, disable two lands. (Tap two untapped lands you control.)
Draw a card.
Enchantment - Aura
At the beginning of enchanted player's upkeep, that player disables a creature. (That player taps an untapped creature he or she controls.)
Pen is Mightier
As an additional cost to cast ~, you may disable any number of creatures. (Tap any number of untapped creatures you control.)
Creatures you control get +1/+1 for each creature disabled to cast ~.
Disable is to be an evergreen keyword action, especially pushed to coincide with the release of a hypothetical "purple" mana set.
Purple mana started as a term for the 6th color in Magic, but over time became synonymous with weird mana or color pie expansion. These example cards are more in the vein of @foo_intherain's idea of "purple"
I sort of wanted something like Exchange, but that's not special since it's just another ability. e.g.
Extra fees - Disable two creatures (As an additional cost to cast ~, tap two untapped creatures you control.)
Disable also coincides with the idea I had for Azorius about tapping enchantments as a cost. Since DJK3654 mentioned Convoke, it seems this is an area worth expanding upon. Cards not related to convoke has had costs that disable one's permanents, usually creatures. It's also similar to Detain, another Azorius keyword action. I'm sure something could work out if disable becomes a part of Azorius' identity. (but not only restricted for a Ravnica-esque block, since disable is meant to be evergreen.)
e.g. Azorius cards with enchantment theme
Creature - Bird Knight
Disable an enchantment: ~ gains flying until end of turn.
Target player disables X creatures.
(Works around hexproof, however opponent gets to choose which creatures gets tapped.)
As an additional cost to cast ~, disable two creatures.
Put five 1/1 white Soldier tokens onto the battlefield.
(has cross-guild synergy with Selesnya and Boros.)