As much as I would love to see Ico, from Ico, or Wanderer, from Shadow of the Colussus, added to the rosters of fighters in Playstation Allstars Battle Royale, I think I have an interesting alternative idea to pay tribute to those games. Granted, I haven't fully thought out the idea or know exactly how to implement them, but I find the idea interesting at least.
My idea is for a new stage where Shandow of the Colossus and the God of War franchise are mashed together. What I would want to see as the stage is that the players are battling ontop of one of the Colossi, and then have one of the Titans from the God of War franchise to appear at some point later in the battle. This could be implemented by having the two fight which would have some dynamic effects to the stage such as a giant fist come slamming down that you must avoid and also destroys parts of the stage. Or it could just be done by having the Colossi wander into the Dessert of Lost Souls where Kronos is wandering around with the temple on his back and the environmental itself is a hazard. Another potential harzard could just be the Colossi shaking due to all the fighters on top of him, causing the plays to lose footing and roll about.
This could even work vice versa where you start off on top of Kronos (or any other titan) and they wander into the land where a Colossi is or to the castle in Ico. I know that this would also give Kratos a third stage, but as far as a mash up goes, I would love to see a Colossi fight a Titan.