Hopeful answer: Cody Hodgson.
As the KesLORD continues his hip surgery rehabilitation (aside: all you Kristen Reid fans will love the last line of Walker's piece) he's clearly saying the right things about being ready opening night for Crosby (maybe) and the Penguins. Then again - considering how vital Kesler has become coupled with the possibility of another 100+ game season ahead - do you risk rushing him back?
So suppose he isn't ready by the season opener. Who steps in at even strength and the first PP unit? Vigneault could shift his depth up and hand the second line to Malhotra while Lapierre takes over the checking line again. This is the safest bet in terms of experience, but expecting Malhotra to produce offensively while shouldering more ES minutes in addition to special teams duties seems like a stretch. It would put more pressure on the Sedins and Burrows to score, but that's hardly the end of the world.
Another scenario is Higgins or Samuelsson take the slot. Both have played center before (though not terribly successfully) and could provide a capable set of hands to keep the line afloat until Kesler returns. Hell knowing Vigneault it's not out of bounds to consider he juggles the second and third lines after the first period each game anyway. So whoever starts in Kesler's slot won't necessarily stay there very long.
There's always the possibility a standout camp audition... Just kidding.
Then there's Hodgson. His star has plummeted in just two seasons, but unless he's moved before October, he seems like a natural to either (a) be tested with the second line to see how he deals with increased minutes and responsibilities or (b) be given the third line while Malhotra anchors the second. It's a risk obviously, but one that might be worth taking if it's just a 5-10 game timeline. The last we saw Franchise he was saddled with piddling fourth line duties which isn't the role he's best suited for. Hodgson isn't cracking the roster as a center without an injury and being down Kesler is his best (only?) chance to step in and make a visible, immediate impact with Vancouver. Alternatively, if he can impress at camp and make it a few regular season games in a top six/top nine role, it's at least an audition for other teams should Gillis eventually move the youngster.