Longer story. Second Son didn't seem very long and was too based around the powers

Most of what has been said already, and Fetch as the main character.

I don't really care for multilayer, but i can see scope for a pretty great MP set up for those who like that sort of thing. I could be interested in a MOBA type MP for Infamous.

Better side missions, the ones in SS suck and are far too repetitive, I think the ones in Infamous 1 and 2 are bad as well, but I don´t remember.

More side missions with more variety.

Longer primary story (with regular ability unlocks and difficulty increases to keep it meaningful).

Power combinations that are meaningful (beyond just melee build up and finisher).

More enemy variety. Require power combinations or sequences to defeat certain enemies to avoid degenerate play (e.g. only using ability X and Y). Consider how the Arkham series gives enemies different weapons and defenses to vary your playstyle from simply Square vs. Triangle. Apply that to ranged abilities and make there be strategies for wearing enemies down or exposing a vulnerability.

More differences between evil and good powers.

Far more differences between evil and good stories.

...

This sounds rather negative. I think it's actually a very fun series with a very visually appealing recent installation with a greatly detailed world, but I feel like it has so much more potential than it taps into. It's good as about 1/4 to 1/2 of an amazing game (in variety moreso than length), but it would be an incredibly iconic game if it diversified its gameplay more. I still think it's worth playing as is though.

a longer game...

Two playable characters, so like Second Son with the add on First Light from day one.

Campaign co-op. That's right, I said it.