There are quite a few discs that can achieve the flight path of a Zone, the problem is that the Zone's rim is pretty unique in hand and that most other discs won't feel like that, for better or worse. As my best comparison, my best friend carries a Zone, while I carry a Rhyno. We use them in the EXACT same areas basically. Their purposes are the same, even if their shape is different. I don't like the Zone's rim, but to each his own. It it saves you strokes, it is impossible to dispute. Main discs I'd recommend if you are looking to duplicate that Zone flight path, but from Innova are (not all putters necessarily):
Roc - Seriously, this disc powered down will fly quite similarly and can be used for other roles as well.
Bulldog - OOP mid that flies dead straight with a hard late fate, much like a Zone.
Pig - I'm a fan of the Thumbtrac. Its not for everyone. Disc flies great and R-Pro plastic stops on a dime.
Rhyno - Basically a slow Pig, and the Pig is a slow Zone to me. My favorite and only OS putter.
Gator - Greater distance potential, but still a straight flight followed by that same dependable fade a Zone boasts.
Cro - The Cro is a straighter Gator to me. Far more laser straight, with a touch of fade after it has really dropped off speed.
Those are about your only options from Innova to fill this role. Any of them can do it, its more about what other uses would you want it for and what feel more sure in hand. For me, it's the Rhyno, but I don't FH my Rhyno. A non-Thumbtrac putter seems easier to grip, and a mid range may fit a FH grip even better still.