Summary
Getting into combo breeding? I made this to see which bases are factors for other bases.

The most common combo factor bases are Ice in first being a combo for 6 bases, Glacial and Arctic tied for second with 5 bases, and Aufeis in third as a factor for 4 bases.

Here's a link to the Combo Bases page on the wiki which you can use to check additional factors like season and time before combo breeding.

Combo factor for 6 bases
Ice (Cairngorm, Elysian, Esker, Hoarfrost, Rime, and Snowflake)

Combo factors for 5 bases
Glacial (Qahir, Esker, Hoarfrost, Rime, and Snowflake)
Arctic (Esker, Hoarfrost, Pulsar, Rime, and Snowflake)

Combo factor for 4 bases
Aufeis (Esker, Hoarfrost, Rime, and Snowflake)

Combo factors for 3 bases
Albino (Cloudburst, Smog, and Snowflake)
Ebony (Elysian, Nadir, and Soul)
Fiery (Citrine, Ember, and Witch)
Flint (Basalt, Ember, and Mudstone)
Frostbitten (Hoarfrost, Rime, and Snowflake)
Opal (Basalt, Celsian, and Qahir)
Unholy (Madagascar, Unholy, and Squall)
Xanthic (Citrine, Witch, and Olive)

Combo factors for 2 bases:
Angelic (Aether and Nether)
Bast (Gilded and Temporal)
Black (Arabica and Zloto)
Blazing (Ember and Pulsar)
Celestial (Cairngorm and Constellation)
Chartreux (Qahir and Cloudburst)
Cinnabar (Black Rose and Citrine)
Cloudburst (Squall)
Cream (Asiatic and Mistletoe)
Demonic (Aether and Nether)
Ennead (Ogdoad and Ra)
Gilded (Rose Gold and Sphinx)
Glass (Celsian and Hexaplex)
Hematite (Celsian and Qahir)
Inferno (Pulsar and Witch)
Interstellar (Constellation and Hoarfrost)
Ivory (Madagascar and Soul)
Jet (Black Rose and Styx)
Nacre (Madagascar and Serruria)
Obsidian (Black Rose and Styx)
Ogdoad (Ennead and Ra)
Onyx (Black Rose and Styx)
Pearl (Cairngorm and Serruria)
Pewter (Ember)
Prismatic (Constellation and Witch)
Ra (Ennead and Ogdoad)
Rose Gold (Gilded and Sphinx)
Russet (Qahir and Zloto)
Sidereal (Cairngorm and Constellation)
Skyward (Cairngorm and Constellation)
Slate (Mudstone and Cairngorm)
Snowflake (Hoarfrost and Rime)
Sunset (Madagascar and Temporal)

Combo factors for 1 base
Alabaster (Rose Gold)
Ambrosia (Constellation)
Anjeer (Pulsar)
Ankh (Ennead)
Ardor (Nadir)
Asali (Mudstone)
Ashen (Majivu)
Augur (Plague)
Black Rose (Nadir)
Blonde (Smog)
Blood Moon (Witch)
Brass (Zloto)
Bronze (Zloto)
Buff (Qahir)
Bushveld (Mistletoe)
Buttercream (Arabica)
Buttermilk (Arabica)
Cameo (Qahir)
Caramel (Olive)
Chaos (Sphinx)
Cherry Blossom (Citrine)
Chestnut (Zloto)
Chocolate (Zloto)
Citrine (Hexaplex)
Citron (Mistletoe)
Cocoa (Cairngorm)
Cream Darker (Mistletoe)
Cream Lighter (Mistletoe)
Dark Vanilla (Smog)
Demiurge (Constellation)
Deshret (Ra)
Dikela (Zloto)
Divine (Elysian)
Duat (Sphinx)
Dun (Asiatic)
Dusty (Majivu)
Ethereal (Cairngorm)
Fawn (Mistletoe)
Festive (Hexaplex)
Fulvous (Citrine)
Green (Mistletoe)
Goldenrod (Hellebore)
Haliotis (Hexaplex)
Hallowed (Qahir)
Harbinger (Plague)
Haruspex (Plague)
Haunted (Witch)
Haze (Majivu)
Heavenly (Serruria)
Hexaplex (Witch)
Horus (Nun)
Gray (Asiatic)
Gregarious (Plague)
Incense (Ennead)
Inpu (Ogdoad)
Kimanjano (Citrine)
Khnum (Nun)
Korat (Cloudburst)
Labradorite (Hexaplex)
Leonid (Constellation)
Lilac (Hellebore)
Locust (Plague)
Maat (Sphinx)
Maltese (Cloudburst)
Maroon (Serruria)
Mauve (Constellation)
Maziwa (Mudstone)
Meteorite (Basalt)
Mobola (Mudstone)
Moonstone (Celsian)
Moss Agate (Mistletoe)
Mulberry (Constellation)
Mummy (Ennead)
Murk (Mistletoe)
Nefer (Ra)
Noctis (Styx)
Orchid (Rime)
Prune (Pulsar)
Ragdoll (Esker)
Reindeer (Esker)
Rhubarb (Constellation)
Rhodonite (Nadir)
Ruddy (Constellation)
Sapela (Madagascar)
Scoundrel (Witch)
Seer (Plague)
Sepia (Madagascar)
Sha (Rose Gold)
Shard (Witch)
Shedua (Madagascar)
Sulphur (Citrine)
Supernal (Madagascar)
Sutekh (Ogdoad)
Swarm (Plague)
Teardrop (Hexaplex)
Thoth (Nun)
Titanium (Styx)
Topaz (Citrine)
Vanilla (Smog)
Velvet (Rose Gold)
Ubaste (Ogdoad)
Udara (Hellebore)
Ukame (Majivu)
Wicked (Witch)
Wepwawet (Ra)
White (Asiatic)
Zarafshan (Zloto)
Zarasa (Zloto)
Zarbanu (Zloto)
Zarin (Zloto)
Zer (Zloto)
Zivar (Zloto)