Critical Hits - When a player rolls a critical hit they may apply maximum damage to their roll instead of rolling another weapon die. This gives criticals a bit more punch. Those classes that add additional dice to their criticals may still do so. Those dice are not maximized (or should the be?).
I haven't decided if monsters should follow this rule yet. I'll play with it in the first session and see how it affects gameplay, one way or the other.
Critical Fumbles - Rolling a one on your attack roll is more than just an automatic miss. It is an opportunity for the DM to introduce additional elements/complications. Broken bow string? Dropped weapon? Additional enemies showing up? I'm going to have fun with this.
Firing a missile weapon into melee - I can't believe there are no rules governing this. I'll allow players to fire into a melee involving one of their allies at disadvantage. If the player misses with the lower die, I'll consult the higher die and see if it hits the armor class of their ally. If it makes narrative sense that the ally could be hit, then the ally will be hit by the missile weapon. Again, will play with this in game and see how it goes.
Playtested this rule last night and it seemed like it was too much of a burden on ranged fighters, which is exactly what I was warned about. Instead of the rule as written above, I'll just introduce the opportunity to hit an ally if the attacking character fumbles his attack roll. The only time a ranged attacker will attack with disadvantage is when he is also in melee and trying to fire his ranged weapon, as written in the rules.
So, thoughts? Am I screwing up the game before I even play?