I agree that it is frustrating for a lot to see that the global event is done with only two or very few players.
From my point of view, the change could be on the travel time to the event according to te size and type of the army.
A small army could travel faster than a big one and much more faster than a Hudge one. Don't you think so ?
Other way to change a litle bit the things : damage equal or greater than 30% total hit points could be reduced to 25% for unavailabling the option for instant attack ?
And may be the more important ; fobide the use of expirering diamonds for instant attack on events
today time to kill dark castle was less than 25 minutes
http://prntscr.com/e36ly5And for sure the events will last longer