Timeline is not the right word. I'm thinking of multiple control channels, as with Director, in which control channels can be turned on and off by keyframe script or other means. A control channel runs like Hype's Timeline actions, but you can create as many control channels as you wish and switch between them.
As for the actions, I am talking about action sequences. Example - mouseUp actions can include setting a flag / watching for a flag. Assign several possible actions to a single object. The flag determines which will run and controls the sequence.
Set the mouseUp to check for flags, then execute an action depending on the flag. Individual actions can have the option of setting/toggling a flag.
Crude flowchart: The Actions UI at present. / Options for stringing individual actions and for combining, as well as stringing individual actions.
(Might as well beat this horse into the ground...) Consider a musicians rig with keyboard, outboard processors, outboard synth and sampler modules, etc - all of which can send / receive program changes as well as direct control of envelope, timbre, delay or whatever "action" is desired. As a MIDI rig you can lay out an entire performance, song by song, with every program change and form of automation you wish. All the musician needs do is to trigger the desired MIDI program and play.
To take things further, tempo must be synchronized for sequenced music and the tempo of delays or filter sweeps. You set a sensor on the kick drum, connect it to the system, and the drummer controls the tempo of the keyboard rig....