My Known combinations:
Water Bottle + Nether Wart = Awkward Potion
Awkward potion + Ghast Tears = Regeneration Potion
Awkward potion + Sugar = Speed Potion
Mundane Potion + Fermented Spider eye = Weakness Potion
Awkward Potion + Magma Cream = Fire Resistance Potion
Water Bottle + Redstone = Murdane Potion
Water Bottle + Glowstone = Thick Potion
Awkward Potion + Spider Eye = Potion of Poison
Awkward Potion + Blaze Powder = Strength Potion
Fire Resistance + Fermented Spider Eye = Slow Potion
Potion of Healing + Glowstone dust = Potion of Healing (Instant Health II)
Potion of Strength + Glowstone dust = Potion of Strength II
Potion of Poison + Glowstone dust = Potion of Poison II
Potion of Swiftness + Glowstone dust = Potion of Swiftness II
Potion of Poison II + Fermented Spider Eye = Potion of Instant Harm
Awkward Potion + Glistering Melon = Insta Health Potion
Adding Gunpowder makes the potion throwable.
If you know more combinations leave a comment!