I'll readily admit that I'm not too much of AGCEEP player (a little too many events for my tastes
at least at some times) and I don't visit here very often.
(so much for the disclaimer)
It seems that what (part of) the discussion is about is:
defining fantasy events. Ie. should an English victory in the HYW be treated the same as a resurgent Byzantine Empire, or a globe conquering Inca Empire.
I think that you should (try to) split it depending on what is "needed" to make the events happen, and when this is/was needed.
For example, the HYW was pretty much still open in 1419 (and, arguably, tilting towards an English victory with Henry V) whereas Byzantium had *in essence* lost against the Ottoman Empire, and lived only because the OE hadn't had the time.
The first was essentially decided IN the EU2 timeframe, the second, before the EU2 timeframe. I know a human player CAN win as Byzantium, and even the AI wins on occassion, and I know that AI England seldom wins the HYW, nevertheless, I think what I said is pretty much true.
That is a first criterion: when is the deviation needed? Before or during the EU2 timeframe?
Secondly: historical plausibility, a tough one, I agree, but certainly one that is needed. For example, the Lollard Heresy; the English kings cracked down on that, but it's not too much of a stretch to think that they would let them be, so long as they didn't become too noisy. (Far) less plausible would be an option like "Embrace the Lollard's teachings, and go on a crusade to spread the Word" - but certainly an option that COULD be worth to explore as a fantasy option. This also ties in with other historical events (for example: "embrace the Lollards" wouldn't fit well with a "take control of the church of England"-conversion to protestant, as you would essentially already have left the Cath. church)
Ofcourse, this will for a large part mean that only the "flavour" of a nation is changed, not the entire nation; instead of an "elite army" there's a "mass army" -- but in both cases there's an army to carry the country's flag.
That's a second criterion: How plausible is it, and how does it fit with other historical events.
A third criterion/consideration would be: how does the AI handle it; I know I (in vanilla) get occassionally quite frustrated, to the point of reloading the autosave, firing events from the console etc. when f.ex. Spain choses to marry a local talent, Spain diploannexes Aragon rather than waiting for the event. Bohemia picking the B choice meaning that Austria doesn't inherit.
Perhaps, rather than making it B (or C) choices in events, have separate events for the human player that give it a chance to "explore" an ahistorical route, rather than having the risk that the AI might stumble into an ahistorical path, without the AI being really suited for it.
Alternatively, you can have AI's in place that will be able to get along with the new situation.
I'm well aware that this is by no means a complete definition, and it's open to arguments/discussions/heated debates/flames.. Nevertheless, I think there's atleast some things that you might want to consider