New Game + is an option in the SR3 save game editor (maybe someone will make a SR4 savegame editor that has it as well).
Having NG+ in game would be fun (set it to one of the consoles in the ship, and boom you are back in time to mission 1).
I'm currently on my 3 or 4th play through.
First time was regular with all side quests, Second was easy with no side quests (deleted that save as I kicked off the giveall cheat and super powered my homies by accident), Second full run was again easy with no side quests.
My current run through is on hard (with the past gang spawn mod installed, and it gets kinda harry at times).
For DLC's:
I would like to see backpacks with working VTOL style flight controls (in either a cape or a set of wings).
And make it so you can select the color of the item in the clothing menu (ala "Lets Pretend").
For the wings have them work similar to the screaming eagle (in hover mode have them flap).
Offer different styles of cape's (with different knock off logo's and lengths).
And more then 1 style of wings (DC's Hawk Girl style, Marvel's Archangel Metal Wings, a set of Demon Bat Wings, a regular set of Angel Wings)...
That would be a DLC work $2.99
note: Weapon options for something like this would be what you have in hand (weapon wheel for primary, super power for secondary).
How about a DLC that offers Transforming options for vehicles into mech suits.
Bikes turn into something like the Iron Saint (Power Armor)
Smaller Car, Truck, and choppers turn into something like the regular Mech
Larger Trucks (Garbage truck), Choppers, and Planes turn into something the size of the Zen Mech Suit
How about More Saints of Rage levels: Relive some of the better parts of SR1 and SR2.
This would call for new levels sets due to the way the camera has to be mapped in the 2d configuration.
Silly Shit add-on's like: Fuzz (I really miss Pirates-vs-Ninja's), Septic Avenger, Vtol Septic Avenger (now you're really slinging shit!), and come up with new fun stuff to kill time with!!!
Oh Assassination Attempts - while walking through VR Stealport someone comes gunning for your character (kind of a reverse to the current Assassination missions).