I'm thinking of extending DiscussionForm with all the Event related form elements.
I can start by
class EventForm extends DiscussionForm
and then write my own event_discussion_form.php file

However, how will i tell vanilla if an event is being added then it should use my version of discussion_form and not the default one.
Plus is this even a good idea, or should i resort to delegations.

Discussion overview actually does this. he (jazzman) references the DiscussionGrid class with his own class. he doesn't however, extend or overwrite the DiscussionGrid class.

Now if i extend the DiscussionForm, and don't overwrite its functions. this way i won't break any extensions that rely on DiscussionForm class.
any thoughts before i attempt to do this.

Btw DiscussionForm should have functions of SaveDiscussion, SaveComment, Reply, etc right now it does everything in the constructor. If I now want to extend SaveDiscussion part of the constructor, I cannot. Something like this would be better

if ($this->PostBackAction == 'SaveDiscussion') {
$this->SaveDiscussion();
}
else if ($this->PostBackAction == 'SaveComment') {
$this->SaveComment();
}
else if ($this->PostBackAction == 'Reply') {
$this->Reply();
}

One more thing what should be type of EventRolePermission field in the database. Should it be a comma separated field like 3,4 which means Moderators and admins, or is their a better way.