I remember a set of feats that expanded what you could do with readied actions. I believe it was two or three feats that let you ready general actions like a move or full round with no trigger to interrupt turn order.
A character with the Overwatch Style feat can ready two separate readied ranged attacks at a time. This takes a full-round action and applies a −2 penalty on each attack.
Overwatch Style has a couple of follow-up feats, Overwatch Tactician that allows for four readied ranged attacks, and Overwatch Vortex which allows you to ready the two ranged attacks as a standard action.
In all three cases, it is unclear if these are specifically readied attack actions, or if these can be any pair of standard actions that involve a ranged attack. The former makes this extremely limited; the latter makes it considerably better (for instance, you could ready Manyshot with each readied action), though considering how many feats it costs and how it messes with your initiative, I have my doubts about it actually being good.
The Cunning Intuition feat allows you to ready actions without having to specify what exactly you will do in response to the trigger. It’s unclear if this is compatible with Overwatch Style (for that matter, as noted, it’s unclear if Overwatch Style gives you any choice in the matter to begin with). In general, probably a superior feat to Overwatch Style overall, but it requires a ton of feats, several of them atrociously awful.
The Well-Prepared trait gives you a +1 bonus to attack rolls and caster level checks made as part of a readied attack or spell, respectively.
The only feat I am aware of that allows you to ready actions without any specific trigger is the D&D 3.5e feat Adventurous Explorer in Dragon vol. 315. It’s reasonable enough to allow in a Pathfinder game, but it’s definitely not official Pathfinder material (it is Paizo material, though, since Paizo published Dragon under license from Wizards of the Coast at the time). The name “Adventurous Explorer” is used in Pathfinder, but it is a trait that just halves the penalties from the shakened and frightened conditions.