Saints Row can no longer be accurately referred to as a "GTA Clone", with the entry IV Saints Row has finished the identity forging that The Third began. Endless customization, wacky and self aware plot; setting; and characters, insane combat/weapons, and countless incentives to experience the world and complete its crazy activities defines the new Saints Row franchise.
Saints Row IV encompasses all these traits and adds to them thanks to the leniency it's digital/extraterrestrial plot allows. The inconveniences of reality are left at the gateway of the simulation, homies and vehicles instantly appear with a phone call and supernatural abilities give you the edge against enemies and traffic. Your new powers make your commute to actives and missions very enjoyable and will likely replace any desire for you to ever enter a standard vehicle again, your powers will also allow you to torture enemies, citizens, and the architecture of the city in whole new ways. Luckily I never felt "too" powerful throughout the game (I played on hard) because the Zin are often able to compensate for your new absurd abilities, and as you conquer more of the city and upgrade your character your experience will become much more enjoyable but the Zin will answer your strengths with more powerful foes as the story progresses. As I said the benefits of your conquest will greatly add to your fun, but the side objectives in your way makes getting those benefits a very fun ride. The riff raff of activities from previous installments have fortunately been replaced by numerous extensions of the mayhem activity and power challenges (excluding insurance fraud); instead of outrunning paparazzi you'll be annihilating opposition with tanks, ufo's, mech suits, blackholes, and your trusty super powers or putting your powers to the the test in rifts, timed races, fight clubs, and the trials of Professor Genki. Hacking shops is forgettable but Zin targets are varied and entertaining; unfortunately most side objectives no longer have the mini stories that the previous games had and contribute to the games lacking story.
Saints Row IV plot has abandoned practically any trace of drama the already lighthearted series once had, and I while I believe every game should at least have something in its writing to keep the player interested the comedy and enthusiastic characters give you enough reason to continue. The story suits the series perfectly but sadly the majority of it is hardly fleshed out. The game starts out well and has strong points throughout but the majority of the plot is presented through calls from your home ship and most optional pursuits lack even that; constant cut scenes aren't necessary to properly present the story but phone calls aren't enough to keep most players enthralled with the story. The inadequacies of the story hinders many of the missions structure as well, resulting many parts of your adventure feel a bit dull.
Saints Row IV is a very fun game with tons of stuff to do that will keep you and your friends very entertained for a long time, but suffers from a weak plot. While the combat and multitude of varied actives are highly enjoyable they suffer from a repetitiveness that doesn't halt your fun but does not keep you dazzled later on in the game. Even with its failings Saints Row IV is a tremendous value that will give you a humorous and lasting over-the-top experience that you will likely give another go to try out the co-op or delve in to the hilarity once more.
Future
So long as Volition is not concerned with realism Saints Row has a very bright future now that they've fully embraced it's identity. Now that the Saints have the technology to travel through space (and who knows what else) opportunities are endless for potential sequels (exploring alien cities and fighting new species would be awesome), but if the developers want a break or no longer feel comfortable with expanding the same story a spiritual successor to the series would be fantastic as well (possibly a fantasy setting?). Either way the next step to advance the outrageous gameplay of Saints Row is to add the environment destruction of Red Faction, and once they combine that with a better fleshed out story fans will be treated to something extraordinary.