rules call on avoiding falling... guys

[Infiniti2000]

In case you are interested, Rules of the Game suggests the following:

Colliding with a Creature: Here's another place where the rules don't help much, so you can use these unofficial suggestions.

You can freely pass through your allies' spaces in the air just as you can on the ground. If you fly into a creature that is not your ally, you effectively attempt to overrun it. You can execute a bull rush against the creature instead, if you wish. An overrun or bull rush normally requires a standard action. If you accidentally enter an enemy's space you must make a Reflex save (DC 15); if you fail, you stall (even if you don't have a minimum forward speed). If you succeed, you can continue with your accidental bull rush or overrun, but you suffer a -4 penalty to all the opposed checks you make to resolve the bull rush or overrun.

As with an overrun attack, the creature can decide not to block your movement, though this might cause the creature to stall (see the section on overruns). If so, you simply move through its space (even if you decide to bull rush the creature). You cannot stop in another creature's square, however, and if your speed isn't sufficient to carry you through the other creature's space, you must attempt an overrun or bull rush.

If the creature is too small to overrun, you must try to bull rush it instead if you can't pass through its space.

If the creature is too big to overrun, you strike it just as if it were an obstacle, and you and the creature take nonlethal damage. Both you and the creature you strike make Reflex saves (DC 15) to avoid damage, but the creature you strike gets a +4 bonus for each size category it is bigger than you. You stall just as if you struck an obstacle. The creature you strike stalls if it fails its Reflex save.

If you are at least three size categories smaller than the creature whose space you are entering (or if you are Tiny, Diminutive, or Fine size) you can enter the creature's space without colliding, bull rushing, or overrunning, but entering the creature's space provokes an attack of opportunity. Likewise, if you are at least three size categories smaller than the creature whose space you are entering, you also can enter the creature's space without colliding, bull rushing, or overrunning, but entering the creature's space provokes an attack of opportunity.

I would give the cast an additional circumstance penalty for being disoriented. The reference to the overruns is the following:

Overrun

A flying creature can plow past or over an opponent using an overrun attack.

Avoiding the Overrun: A flyer can avoid an aerial overrun just as a landbound creature can. If the creature has a minimum forward speed, it must make a Reflex save (DC 15) to avoid stalling after the sudden maneuver. If the defender decides to avoid, you can move through its space whether it stalls or not.

Blocking the Overrun: If the opponent decides to block, make opposed Strength checks or opposed Dexterity checks (each creature involved chooses which to use).Bonuses and penalties for size are the same as a normal overrun no matter which kinds of checks the opponent's use. In addition, each creature gets a bonus based on its maneuverability rating, as follows: perfect +12, good +8, average +4, poor +0, clumsy -4.

Stability bonuses do not apply in aerial overruns.

Overrun Results: An aerial overrun generally has the same results as a regular overrun, except that a creature knocked prone stalls instead (even if it doesn't have a minimum forward speed). If an overrunning attacker wins the opposed check, it can inflict unarmed strike damage on the defender instead of making the defender stall. A creature with natural weaponry uses the damage rating of one of its primary attacks as the unarmed strike damage.

 

[evilbob]

These are all excellent suggestions, thank you.

I will say that personally, the reason I avoided things like grapple checks and overrun attempts is because casting dimension door is a standard action. You really don't have any action left on your part (other than, well, the movement), so technically you can't reach out to grab someone (an attack) or attempt to push past or bull rush them (also standard actions). This makes even more sense when you add the "disorienting" effect of dd as some have mentioned.

Overall, I think the reflex check on the part of the "avoider" with a DC of 15 and a modifier of +/- 4 per size category difference of the creatures involved seems to be more in line with the "rules of the game" post and my own idea in the OP.

(Plus, this leads nicely to what I always thought would be a great high-level druid tactic: a) wildshape into a sparrow, b) fly above enemy, c) wildshape into a sperm whale.)