Currently, the Ready Basic Activity uses up three out of four of a character's total actions per turn (2 during their turn, plus your Reaction) in order to properly time one action. Losing two actions is an awful steep price to pay and severely limits readied options. I feel the ratio of using two actions to properly time 1 action makes more sense.
This could be accomplished by making Ready a single Action instead of an Activity, but I feel the opportunity cost of spending two actions on your turn is an important factor to balance the timing option Ready gives. This also still severely limits readied options, as we can only ready one Action and we lose any of the nifty class ability Reactions we get.
A better option would be to have the two Action Ready Activity grant an additional Reaction. This allows for an interesting array of options. A Paladin could Raise a Shield and Ready to Attack and get both the readied Strike and a Shield Block. Your Fighter standing next to a doorway could get his readied Strike and if the enemy tries to then keep moving take his AoO (and maybe a second AoO on the next enemy that wanders in, if he has Combat Reflexes). A cleric could Ready a touch range Heal and still be able to use Divine Ward just before using the Heal spell.
Another interesting option this opens up is a Ready which can be two actions. I haven't pored over all the possibilities, but spending 3 actions to get 2 as well as the opportunity cost of losing two of those actions on your turn still seems like a reasonable cost to me (admittedly it is half the cost of the current design, but I think the current is significantly too steep).
A Paladin could Raise a Shield and Ready to Attack a creature she expects to move into melee and get both the readied Strike and a Shield Block. But the enemy instead moves to a spot out of reach of the Paladin, but within a stride. The Paladin could use her regular reaction to Stride into melee and then the Ready can now trigger (or maybe an ally moved the enemy into Stride range). Of course the Paladin no longer has any reactions to Shield Block nor Retributive Strike.
The Fighter holding a melee weapon who readied to attack an enemy that comes in range can now opt to Sudden Charge as soon as the enemy gets within that range, rather than just melee range, but without other Feats, can no longer AoO.
This also opens the possibility to ready two action spells. You could Tanglefoot or Acid Arrow something when it first comes into range. Throw a Fireball into a room once the door opens. Range Heal if prepared.
Not sure the two action ready is viable, but it is an interesting thought.
