I like those games. The karma system is a little too stereotypical and black/white. It rewards being the extreme of either side but not if you play more naturally. I hope that in future games, they use a vastly more nuanced system on morality.
It is to facilitate different gameplay styles. I rather have that than just some different dialogus and same gameplay in both sides but many people seem to not want more gameplay variety, which is what I love about this series.
All I want from another inFAMOUS (if we ever get one, which I unfortunately doubt) is inFAMOUS 3, set after 2's evil ending. It would be set in a world where conduits and normal human beings are at an all out war as the conduits, with Cole as their leader, advance from the US east coast towards the west coast as Cole cures inactivate Conduits from the plague as normal humans simply die out in the process. The game could be set in a city like Dallas the moment the Conduits arrive. The moment the game starts the city isn't damaged, although it's heavily militarized to try and stop the Conduit advance. As the game progresses you (Cole) and your Conduit allies slowly take control of the city, causing human life to disappear and the city to get more damaged by the day.
I'm sure that kind of a game wouldn't be well recieved by audiences when you're playing as the bad guy.