The Slayer might be my favorite of the new classes, but the big factor to making it the kind of class I want to revisit and approach from different different directions will be a healthy variety of flavorful Slayer Talents. (Also on my wish list would be making the Slayer Talents available to Rogues the same way that Ninja tricks are, but not making all the Rogue stuff available to Slayers. I think it's in character for Rogues to be more option-based and Slayers be more focused).
The flavor of the mechanics and skills the Slayer already has lends itself to:
-Studying your enemies to gain an advantage.
-Lurking/stalking your prey.
-Ambushing from stealth.
-Being implacably focused on a single enemy.
-Being frikkin' scary.
Plenty of cool options to pursue- you could basically watch Batman: The Animated Series for a few hours, write down everything cool that Batman does, and make each one a talent. A few possibilities:
-Let the Slayer Pounce its favored enemy and get Sneak Attack on it's attacks if and only if the Slayer has concealment.
-Bonus to attack and damage if the Slayer is attacking while jumping down onto its target from an elevated position.
-Access to Ranger Traps, and making the traps more effective against the Slayer's Favored Enemy.
-Giving the Slayer make an Intimidate check to Demoralize their Favored Enemy when the Slayer has concealment and the target is unaware of the Slayer's presence. (The target can't put their finger on it but they feel like something is watching them...)
-Eliminating penalties to Stealth and other skill checks while wearing Medium armor.
-Letting the Slayer charge their Favored Enemy as an attack of opportunity if the Favored Enemy attacks someone other than the Slayer. This would combine nicely with the Stealth Pounce trick and let the Slayer simultaneously be stealthy and still do some of the "protect my buddies" job that full BAB d10 HD classes traditionally take care of.
-Letting the self-heal when they defeat their Favored Enemy. This would help justify ignoring other dangers and absorbing the occasional hit in favor of taking down the FE.