As some of you may know, I'm not running D&D, I'm running a game called Monsters & Magic (M&M) which I made myself, And M&M doesn't have the same kind of rules as D&D, It's more freeform. I need help coming up with cool and interesting potions for a potion shop I'm making, The potion shop allows you to mix any two potions as long as you pay an appropriate amount of money so I'm looking for potions that would combine with other potions well and create interesting effects, The potions I currently have are: Three different strengths of Healing potions, Four different kinds of elemental potions (Which allow you to control that element), An antidote potion (Cures certain status effects), And a splash potion (Which does nothing on it's own but if you combine it with another potion then it gives that potion an AOE effect), Any ideas anyone?

Hey, I don't know what rules may be similar to D&D5e in M&M, but as someone who does still homebrew to large extent and run a high magic campaign, I think that sounds like a cool idea!

Some ideas:

Haste potion. Maybe either have it double movement speed or allow a non-offense/non-spell bonus/free action per turn for ____ amount of time.

Potions of Charm/Vitality/Strength/Etc; allow player to roll certain skill checks at advantage/+a certain amount, for an amount of time, depending on what suits your game's rules.

Some less standard ones like eyedrops of darkvision, Dragon Breath (combine with the elemental potions for different effects), Potion of Familiar (allows two creatures to interact as if they are a Wizard and a Familiar for a certain amount of time) or potions of comprehend languages would be cool, to me - anything that offers a Baby Level version of a different class/racial benefit.

That last concept kind of also digs in closer to the world at its core; what kind of potions would the people in your world MAKE, if they're able to do this kind of brewing?

I like the Haste potion and Strength potion idea, I think I'll use them, Also this whole ”Potion mixing” thing is almost unique to this shop, Not many other people have the skill, Expertise and materials to do this.

Water Breathing (can be combined with fire for fire breathing, which is a very different thing)

Invisibility (can be combined with any element to make you "invisible" to that element- with Earth, allows for a passwall type effect, walking through walls, with water, makes you sink like a stone and walk normally on the bottom of the body of water, and with fire, makes you immune to damage from it)

An inverse potion (makes a potion do the opposite thing instead- turns healing potions into poison, makes antidote cause status ailments, and with a splash potion creates a mini vortex)

Omg, Those ideas are amazing, I'm probably gonna use all of them (Except for the water breathing because my PCs have water walking boots and there's nothing that they’d want underwater)! But I don't get the combinations of the invisibility with water and fire, Why does being invisible turn to making you sink like a stone and be immune to fire damage?

The idea is that you're invisible to the element, so it doesn't interact with you.  For water that means you can't swim in it, it doesn't touch or interact with you in any way, it's like the water isn't even there, so you're just walking along the bottom.

Similarly the primary effect of fire interacting with you is taking damage from it, so if the fire does not interact with you, you don't take damage.

It's a loose concept and interpretation of "invisibility", almost along the line of the idea of invisibility being more like etherealness, but hey, it's all just thoughts, if you feel like modifying it or discarding it that's 100% ok.  

Would the Invisibility to Earth potion cause you to sink to the planet's core? Seems dangerous!

I once made a potion that induced Wild Magic. It was basically how the sorcerer in that campaign got their powers.

But you could so something similar for a potion mixture that has gone wrong maybe? So if people mix two effects that just don't work with each other (eg fire and ice) it causes some chaotic magic effects as a result. You could just make a small table of possible effects that could happen and roll on that when it happens. If you wanted to save time you could even use the Wild magic table from 5e as a bases fairly easily.

But it could be fun for some random chaos and adds some risk to the mixture process if you like the idea of that. 

Potion of Endurance. Gives the benefits of a short or long rest depending on whether you have an endurance or stamina mechanic in your game. Could add a crash effect for when the potion runs out.

Potion of Darkvision. Gives Darkvision for a limited amount of time. Combined with a Potion of Invisibility, allows imbiber to see invisible.

Potion of Purity. Turns poisonous or unhealthy food/water into edible food/water. When combined with another potion, it turns it into pure water.

Potion of Concealment. Can be applied to self or objects to make them unnoticed. Basically giving things a Stealth roll of X.

Potion of Perception. Increases Passive Perception by X amount.

Sorry Wysperra, but sadly none of those potions would work with the mechanics I have in M&M, Also you used the word ”Imbiber” wrong, It means to drink alcohol, Not just to drink anything