Annotations are definitely one solution, but until they become a core PHP solution using them is going to be clunky (I integrated Doctrine ORM with Annotations support into a Framework based project, getting the configuration right for that was rather interesting).
Having events scattered throughout the system would definitely be a +1 from me. It's something we're really missing right now, you have to override anything to add any sort of event/hook based system into an application; it should be something we aim to support in the API.
I'm having a hard time putting to words what's in my mind right now, but I think we really should start experimenting with code and getting a feel for where we want parts of the API to go. In the case of the Form package, I'd almost suggest just starting fresh with it, splitting it into logical components. There's a validation piece, rendering piece, and the primary API minimum; that API should be hookable in some form (events presumably being one of the easier ways to get started with that). We can dispatch events with ease, but registering event listeners is something that's a little tricky right now. So how can we make it a bit easier to add event listeners into a dispatcher? Symfony lets you do it through either the DI Container (which gets compiled and cached) or using event subscribers. The public API only supports simple arrays for the data store, how can we make that work with perhaps our DataObject
or other types of objects? I think the Form package can become a model of a well architected API and experimenting with it would be rather interesting.