The reason it’s hard to figure out what to put in some hypothetical Episode 2.5 is because the whole Prequel series was poorly structured and it’s hard to figure out climaxes or cliffhangers to bridge the episodes. Ideally, Phantom Menace shouldn’t exist, and instead some kind of “Intro to Clone Wars” plot line should be Episode 1. Episode 2 should depict one or more critical missions during the Clone Wars, and Episode 3 should be either after the Clone Wars or during the end of the wars.
But there’s usually a rhythm or pattern in a Star Wars trilogy. The first episode ends with a triumph for the good guys. The second episode ends in defeat. And the third episode ends with a final triumph. Since the Prequels have to end in disaster, perhaps the Prequels should mirror/inverse that pattern: so we get a defeat, then triumph, then final defeat/disaster. So that pattern would suggest that Episode 2 would depict a major turning point in the Clone Wars where the (ostensibly) “good guys” get a major victory.