I'd just worry about getting the basics implemented... Basic Melee Attacks, Basic Ranged Attacks, Move Actions, and Opportunity Attacks. Once you've got that done, it's just about adding triggers and new hooks on those triggers.
Well, it's a little more complicated than that. Basic attacks & OAs are almost trivial to implement, because they don't really do much of anything aside from damage, and they aren't going to trigger additional interrupts. The meat of the implementation is what happens when an event can trigger multiple immediate actions, each of which can (a) change the conditions of the initial event, and/or (b) trigger more immediate actions. And somewhere in the middle there one of the events gets invalidated by someone else's interrupt.
Speaking of which, and getting back to the OP, I have a rules question. I know how I would play this at the table, but I am unsure as to what the RAW say on the topic.
A dragon breathes on 3 PCs.
PC1 is about to be hit, but uses an encounter interrupt to boost his defenses, turning the hit into a miss.
PC2 gets hit and takes damage.
PC3 is hit and is about to take enough damage to drop, but uses an interrupt to make an attack against the dragon. PC3's attack kills the dragon, which invalidates the attack...
...invalidates it against PC3? Or against all three PCs?
The dragon is dead and PC3 took no damage from the breath.
Does PC1 still have his or her immediate action? Is the encounter power expended? Is PC2 still damaged by the attack? Would the PCs be better off if the attack vs PC3 was resolved first?