On a hunch, we could just be flexible with initiative, so the non-rogue could choose to attack first.
(Problem: Two rogues cannot benefit both from flanking.)
Or we exchange Flanking with the 'Combined Attack' action, that gives two or more characters who attack the same target a +2 to attacks and possibility for sneak attacks.
Just my spontaneous thought. But this is why we want to do a playtest.
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I don't think it is overpowered. If one doesn't mind AoO, it isn't hard to get into flanking position.
New Acrobatics use:
You can move to flank an opponent without provoking an attack of opportunity from an enemy by using Acrobatics. You cannot use Acrobatics for this, if your speed is reduced due to carrying a medium or heavy load or wearing medium or heavy armor. If an ability allows you to move at full speed under such conditions, you can use Acrobatics to move past foes. You can use Acrobatics in this way while prone, but doing so requires a full-round action to move 5 feet, and the DC is increased by 5.
Code:
Situation Base Acrobatics DC*
Move through a threatened area Opponent's Combat Maneuver Defense
Move through an enemy's space 5 + opponent's Combat Maneuver Defense
* This DC is used to avoid an attack of opportunity due to movement. This DC increases by 2 for each additional opponent avoided in 1 round.
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The Cthlhu d20 variant just got rid of AoOs, but forced to stop if the movement would incur one. So, you could simply require a double move (including the 5ft step for the threatened sq) to move in flanking position without a roll.
And mobility could be upgraded to just allow for moving not incurring AoOs.