Alchemist is fairly easy to fix IMO. The main problem is that they scale poorly.

  1. Keep the existing Experimental Elixirs and scale them all whenever Cantrips scale. Movement infusions increase by 5ft at 5/11/17. AC infusions add +1/+2/+3/+4. Boldness adds a +1/+2/+3 bonus. Healing advances to 4d4/6d4/8d4. Alter Self becomes Polymorph. etc.
  2. Add several potions as infusion options via Replicate Magic Items and bonus infusions known that can only be used on potions.
  3. Give them some extra daily infusions that can only be used on potions.