These are for a cube with a ton of cards that are modified or combined versions of existing magic cards as opposed to "from scratch" customs or "as printed" cards. Also, note these are for a cube that is aiming for a relatively high (but flat) power level and likely don't make sense slotted into lower power lists.
Turnabout / Sleep
At the end of Turnabout, add "If it's your main phase, those permanents don't untap during their controllers next untap step" to make it act as both a storm card and a tempo card (similar to Sleep).
Lion's Eye Diamond / Lotus Bloom
Smash these together by making a Lotus Bloom with "Discard your hand: Remove all time counters from Lotus Bloom."
Lotus Petal / Mox Diamond
Change Mox Diamond's sacrifice clause to "at end of turn" so it can act as a Lotus Petal when not discarding a land.
Mox Opal / Chrome Mox
Both abilities combined on one card.
Jeskai Ascendancy
Change cost to to match colors that most support storm.
Sarkhan Unbroken / Dragonstorm
Lower ultimate to [-7], widen from library to "library, hand, and graveyard," and change from "put onto the battlefield" to "cast without paying mana cost."
Scapeshift
Change to "untapped" to enable use as ritual
Accumulated Knowledge
Draft one get three.
Rite of Flame
Draft one get three.
Memnite / Arcbound Worker
Costs , Modular 1, Draft one get three.
Metalworker
Birchlore Rangers
"Tap two untapped creatures to add 2 mana of any color."
Nettle Sentinel
2/3
Nivmagus Elemental
2/1
Gitaxian Probe
Instant
Noxious Revival
"top or bottom" (more playability as incidental graveyard hate, also miracle enabler)
Cloud of Faeries
flash, "tap or untap two target lands"
Etherium Sculptor / Master of Etherium
Noncreature artifacts you cast cost less. ___'s power and toughness are each equal to number of artifacts you control. Other artifact creatures you control get +1/+1
Etched Oracle
Costs , remove from activation cost
Mana Bloom (also enchantress support)
Remove "activate this ability only once each turn"
Beck // Call (also token / enchantress enabler)
Change to Instant costed and and creates 3 2/2 enchantment bird tokens.
Wargate / Chord of Calling
Instant, convoke, put any permanent with cmc from library onto the battlefield.
Shardless Agent
Give flash so it can hit counterspells in addition to being a 2 storm count for 1 spell card
Ardent Plea (also a devotion and control/loner enabler)
Change cost to
Channel the Suns
Change cost to and add draw a card to make it a mega manamorphose
Mindbreak Trap
Change cost to to make it more useful outside of just storm hate
Necropotence
Change cost to and remove graveyard exile clause to make it more playable in graveyard dependent strategies and easier to splash.
[Non-storm side note: Seismic Assault also changed to to complete the Spectral Procession (which is changed to an instant) cycle]
Storm Entity
Trample
Actual storm cards
Brain Freeze
Crow Storm
Empty the Warrens (Instant)
Grapeshot (Instant)
Plus other prowess / spells matter stuff and other things I'm not remembering/finding right now...
Turnabout / Sleep
At the end of Turnabout, add "If it's your main phase, those permanents don't untap during their controllers next untap step" to make it act as both a storm card and a tempo card (similar to Sleep).
Lion's Eye Diamond / Lotus Bloom
Smash these together by making a Lotus Bloom with "Discard your hand: Remove all time counters from Lotus Bloom."
Lotus Petal / Mox Diamond
Change Mox Diamond's sacrifice clause to "at end of turn" so it can act as a Lotus Petal when not discarding a land.
Mox Opal / Chrome Mox
Both abilities combined on one card.
Jeskai Ascendancy
Change cost to to match colors that most support storm.
Sarkhan Unbroken / Dragonstorm
Lower ultimate to [-7], widen from library to "library, hand, and graveyard," and change from "put onto the battlefield" to "cast without paying mana cost."
Scapeshift
Change to "untapped" to enable use as ritual
Accumulated Knowledge
Draft one get three.
Rite of Flame
Draft one get three.
Memnite / Arcbound Worker
Costs , Modular 1, Draft one get three.
Metalworker
Birchlore Rangers
"Tap two untapped creatures to add 2 mana of any color."
Nettle Sentinel
2/3
Nivmagus Elemental
2/1
Gitaxian Probe
Instant
Noxious Revival
"top or bottom" (more playability as incidental graveyard hate, also miracle enabler)
Cloud of Faeries
flash, "tap or untap two target lands"
Etherium Sculptor / Master of Etherium
Noncreature artifacts you cast cost less. ___'s power and toughness are each equal to number of artifacts you control. Other artifact creatures you control get +1/+1
Etched Oracle
Costs , remove from activation cost
Mana Bloom (also enchantress support)
Remove "activate this ability only once each turn"
Beck // Call (also token / enchantress enabler)
Change to Instant costed and and creates 3 2/2 enchantment bird tokens.
Wargate / Chord of Calling
Instant, convoke, put any permanent with cmc from library onto the battlefield.
Shardless Agent
Give flash so it can hit counterspells in addition to being a 2 storm count for 1 spell card
Ardent Plea (also a devotion and control/loner enabler)
Change cost to
Channel the Suns
Change cost to and add draw a card to make it a mega manamorphose
Mindbreak Trap
Change cost to to make it more useful outside of just storm hate
Necropotence
Change cost to and remove graveyard exile clause to make it more playable in graveyard dependent strategies and easier to splash.
[Non-storm side note: Seismic Assault also changed to to complete the Spectral Procession (which is changed to an instant) cycle]
Storm Entity
Trample
Actual storm cards
Brain Freeze
Crow Storm
Empty the Warrens (Instant)
Grapeshot (Instant)
Plus other prowess / spells matter stuff and other things I'm not remembering/finding right now...