That's... actually pretty reasonable. (Although I might word it "you fall at the end of your next action." Immediate/unless just seems kinda murky and open to interpretation.) Would you then be allowed to use any power while jumping, or do you intend to only allow ones that involve movement? Including powers that allow movement after the attack?
I'd be inclined to allow any power, and have the character fall then (unless he has since reached solid ground).
Aside: if you roll high enough to jump 6 squares, but are slowed, can you (RAW) use an action point to TRIPLE move and complete the jump? Should you be able to?
I don't see why a "triple move" shouldn't be allowed. Indeed, for an Epic level character with optimised Athletics, this may be the
only way he can use his full jump result!
That said...
I'm starting to think that maybe the Athletics result shouldn't
directly control the distance the character can jump at all - as someone mentioned up-thread, that's actually a bit too swingy.
It might be better to have the jump DC set by environmental conditions (per Page 42), with a success allowing the character to jump half his speed on a success. (Probably include a clause that "for every 10 points you beat the DC, you can add 1 square to your movement" - I'm in favour of adding that clause to
every Athletics-based movement, including running.)
(Note that this more or less matches climb and swim, where the DC is environmental, and success doesn't change the rate of climbing/swimming. I do like consistency.
)
Note that this would mean that a running jump
does not change the DC - running increases the characters speed for the purpose of the movement.
This would also seem to fix my issues with "too easy" Olympic jumps at 1st level - if we assume the World Record holder has a Str-equivalent of 18ish (making PCs with a 20 somewhat fantastical by their very nature), trained in Athletics, and with the appropriate Skill Focus, he has a +12 modifier. If the DC for an Olympic Jump is something really easy (say 5 - reflecting the fact that they are totally unencumbered, are wearing lightweight footwear, and aren't busy fighting for their lives), then they can hit between 25 and 35 on the check with some regularity. Because they're running, their speed is effectively 8, so the standard jump is 4 squares +2 for hitting 25 - which is almost exactly the 30 ft of the World Record.
It does mean that a standing jump of 3 squares is probably too easy, but hey - can't have everything, right?
(Hell, add a clause that
any jump where the character doesn't move 2 squares first has a -5 penalty, and that fixes that, too.)
(Oh, yeah - sorry for going on way too long about jumping. I think I've finally managed to say everything I have to say on this topic now.
)