Here's my take - using the "remove" trick and piggybacking the "riddle me this/immediately" trick by moving it to the start of the turn:
TURTLE POWER
After revealing an Order Marker on this card, before taking a turn with Leonardo, you may immediately remove up to 3 Order Markers on other Ninja Turtle cards to take one turn each with those figures.
Seems like it addresses the issues. You might need a FAQ about how it interacts with self-importance, but I assume the idea would be to let the turtle power turns play out, then roll for SI to see if Raph takes a turn instead of Leo.
(As an aside: how bad Self-Importance is in general is a function of how valuable the turn with that figure is. If Thor had self-importance, nobody would really care.)