Saints Row 4 ends with the player character and his team becoming a universal power, In any other case, this would be the end of the line. But both this AND the third game show one thing. The player character is terrible when it comes to being a
1.role model
2.strategist
3. leader of anything organized.
Not to mention that he is great at making powerful enemies. The entire 4th game showed that he still was willing to do stuff on his own. And, judging by the note that the third game ends on, Killbane is more than likely to have survived the demolition of Earth)
However, the super powers feature could add more than just grenade replacements ( more Melee combat would be nice, also nerfing the sprint a small amount)
and the setting of the game is freaking space.
Saints Row, a game that relies on crazy enemies and being meta as all hell, just went into SPACE.
Another thing to understand is that gameplay is more than just features, it's also level design and physics.
That's where movies differentiate from games in my opinion, with movie series, you can only go so far before you stop innovating, and start straying from the concept (Jason X, anyone?), eventually, you just have to look at the legacy you've created, and say "I did good", before you end up turning it to crap (Star Wars original trilogy vs prequel trilogy)
With video games, story and gameplay go hand in hand, certain elements of story allow the developers to add various things to the gameplay. As many fans of the reboot have also pointed out, as long as you stay true to the core gameplay, it's okay to remove a few features in order to replace them with different mechanics, as the series can always switch back. That's PART of innovation