Admin Edit: The following is speculation, and should not be considered canonical. - Shiv
Malatium
Composition: Atium & Gold
Allomancy: Medium mistings can burn malatium to see the possible pasts of other people.
Feruchemy: Malatium stores reflexes. Quicksilver ferrings storing it become extremely slow to react. Tapping it heightens reflexes significantly.
Electratium
Composition: Atium & Electrum
Allomancy: Prophet mistings can burn electratium to see a possible far future of themselves or of a target they're touching.
Feruchemy: Electratium stores skills. Repeater ferrings can watch others performing non-magical actions and store them. Tapping it would make them remember those skills, and be able to perform them. They would need to store them again after use or they will be forgotten.
Bonatium
Composition: Atium & Steel
Allomancy: Repulser mistings can burn bonatium to automatically repel every metallic object in their vicinity, at 360 degrees. The lighter the metal, the more it's repelled. The heavier it is, the less. Metals heavier than the allomancer are unaffected.
Feruchemy: Bonatium stores gravity. Mass ferrings storing it are much more affected by gravity, making it hard for them to move. Tapping it makes gravity affect them less, which causes them to walk as if on the moon.
Featium
Composition: Atium & Iron
Allomancy: Magnet mistings can burn featium to automatically attract every metallic object in their vicinity, at 360 degrees. The lighter the metal, the more it's attracted. The heavier it is, the less. Metals heavier than the allomancer are unaffected.
Feruchemy: Featium stores density. Juggernaut ferrings storing it become very fragile. Tapping it makes the body much more resistant to damage, to the point of invulnerability.
Spealtium
Composition: Atium & Pewter
Allomancy: Reaper mistings can burn spealtium to make targets sick by touching them. Continuously touching targets would eventually kill them.
Feruchemy: Spealtium stores tranquility. Tranquilizer ferrings storing it become angry. Tapping it makes them relaxed.
Snatium
Composition: Atium & Tin
Allomancy: Healer mistings can burn snatium to heal targets by touching them.
Feruchemy: Snatium stores light. Nightshade ferrings storing it would see everything darker. Tapping it would make them see better in the dark, to the point of seeing in complete darkness.
Znatium
Composition: Atium & Zinc
Allomancy: Exciter mistings can burn znatium to enflame (riot) their own emotions.
Feruchemy: Znatium stores persuasion. Persuader ferrings storing it wouldn't sound credible. Tapping it would make them more persuasive.
Bratium
Composition: Atium & Brass
Allomancy: Sedater mistings can burn bratium to dampen (soothe) their own emotions.
Feruchemy: Bratium stores interest. Keener ferrings storing it would find everything boring. Tapping it would make them find everything interesting.
Latium
Composition: Atium & Bronze
Allomancy: Finder mistings can burn latium to detect feruchemical use. It is possible to dectect the metal being used, what is stored, what is tapped, and how much.
Feruchemy: Latium stores x-rays. Watchman ferrings storing it become blind. Tapping it enables them to see through everything except for dense metals such as lead.
Cuatium
Composition: Atium & Copper
Allomancy: Concealer mistings can burn cuatium to form a bubble that protects feruchemists from being detected by finders. A side effect of the burn is protection to the allomancer from seers, mediums and prophets.
Feruchemy: Cuatium stores camouflage. Lurker ferrings storing it would become massively noticable. Tapping it would make them blend with the environment, to the point of invisibility.
Duratium
Composition: Atium & Duralumin
Allomancy: Mistborns can burn duratium to shield themselves from nicrobursts and leechers. Duratium mistings are called Duratium Gnats because burning their metal is useless.
Feruchemy: Duratium stores linguistics. Linguist ferrings storing it wouldn't understand anything being told and wouldn't be able to read, speak or write. Tapping it would allow them to understand, speak and write any language, both spoken and written.
Alatium
Composition: Atium & Aluminum
Allomancy: Mistborns can burn alatium to fill all of their other allomantic reserves, even if they're empty. The more alatium being burnt, the more the reserves fill. Alatium mistings are called Alatium Gnats because burning their metal is useless.
Feruchemy: Alatium stores wisdom. Prodigy ferrings storing it would become dumb. Tapping it would make them smarter.
Cratium
Composition: Atium & Chromium
Allomancy: Charger mistings can burn cratium to fill allomantic reseves of a target they're touching. The more natium being burnt, the more the reserves fill.
Feruchemy: cratium stores reading. Reader ferrings storing it would only be able to read very slowly. Tapping it would enable them to speed-read.
Natium
Composition: Atium & Nicrosil
Allomancy: Screener mistings can burn natium to form a bubble that shields people within it from emotional allomancy.
Feruchemy: Natium stores voice. Holler ferrings storing it would have their voice quieter. Tapping it would make their voice much louder.
Cadatium
Composition: Atium & Cadmium
Allomancy: Jumper mistings can burn cadatium to teleport a short distance. If all the reserve is burned out instantly (such as by being touched by a nicroburst), it is possible to teleport anywhere in the world.
Feruchemy: Cadatium stores molecular integrity. Ghost ferrings storing it are put into stasis. Tapping it enables them to move through solid objects.
Benatium
Composition: Atium & Bendalloy
Allomancy: Synchronizer mistings can burn benatium to slightly slow down time for everything and everyone other than themselves and other synchronizers. If all the reserve is burned out instantly (such as by being touched by a nicroburst), time stops completely for a few seconds.
Feruchemy: Benatium stores euphoria. Slosher ferrings storing it feel withdrawal symptoms such as from drugs. Tapping it makes them feel euphoric, as if on drugs.
Bonus:
Leratium
Composition: Atium & Lerasium
Effect: Ingesting leratium makes one a full feruchemist.