If you look at the properties of common herbs, you should be able to come up with something on your own. Just be sure to take into consideration both the positive and negative effects of each.
For example: Sassafras has been used for medicinal uses for many, many years. This includes cleansing toxins from the blood, thinning blood to aid high blood pressure, acting as a mild diuretic, as well as several other lesser effects. However... Scientific research has led the FDA to ban it's commercial use in the United States for fear that its use may lead to liver cancer if taken in high doses or taken frequently.
If combined with an herb that has very mild side-effects that, in turn strengthen the liver, there should be less risk in using it (Although, be warned that there is ALWAYS a risk).
Use your own discretion, as creating potions takes research and patience, as well as a working knowledge of herbs, the medicinal effects of each, as well as their negative side-effects.
I suppose a safe potion that has been touted to have a wide-variety of possible effects (although they don't work reliably on everyone) is a mixture of cinnamon and honey (1 tsp each) mixed with 8 oz. of boiling water.
That mixture is said to aid respiratory functions, aid weight loss, aid with a sore throat, and many other things (you can research it more if you'd like to try it. You'll discover proper dosages through research as well). It is safe to use on anyone that does not have an allergy to either of those ingredients, EXCEPT FOR YOUNG CHILDREN!!! Refrain from giving anything with honey in it to children younger than 1 year old. It has detrimental effects on their development
Also... if you define a potion as something that is brewed from plant matter that has medicinal benefits, both coffee and tea fall into that category. Those are probably the easiest potions you'll ever make :P (Just make sure not to add sugar)