I think that it would be cool to be able to make Potion of Flame or Potion of Withering to use to fight with. The Potion of Flame would upon contact would cause an AoE fire damage affect that would set any entity in range on fire, to make it you could use fermented spider eye on a Potion of Fire Resist to make the Potion of Flame
With Potion of Withering's recipe, maybe add a rare drop from a Wither Skeleton called a Withered Bone witch can be turned into Withered Powder witch can be used to make the Potion as well as use a fermented spider eye on the potion to make a Potion of Wither resistance, the Potion of Withering would upon contact cause an AoE Wither affect on any entity in range.
The range of the Potion of Flame and Potion of Withering would be the same as any other splash potion.
I think that it would be cool to be able to make Essence of Fire or Acid Flasks to throw, and cause more damage than the "Splash Potions of Harming".
How would they be made? How much damage is dealt?
I also think that the range that a splash potion should be increased so that the player can't hit themselves with their own de-buff splash potions.
What do you mean...? Increase the throwing range? I think a huge risk of using splash potions is how you can get the negative effect, which is what I like. It balances it out because you have to decide whether or not to use it.
Maybe add a slingshot that can fire the splash potions much farther than throwing them.
Those are my ideas for more Alchemical based combat.
If you want to throw them farther, then get potion arrows. Though, they do require lingering potions...how would this be made?
Overall, I can't support the second idea. The other two are really vague.