He'll be back in the NHL and he'll produce. It probably won't be with us and he probably won't be as expensive at what some might think. Almost no chance he's be back to scoring goals, but 25 seems quite reasonable. What's that worth, for how long, with a known flight risk? Who knows.
This post brings up the "how long" question which in Kovalchuk's case I think has to be one year.
He's about to turn 35 so if he is signed for multiple years, it will be a dreaded age-35 contract in which in the second and subsequent years the team has to take the cap hit regardless of whether or where he's playing. So if in Year 2 of a deal Kovalchuk decided to retire from the NHL and return to Russia (sound familiar?) then the team that signed him in the NHL would still have to include his salary and bonuses in accordance with Article 50.2(c)(iv) of the Collective Bargaining Agreement.
Between the questions of when will he regress due to age, how good will he still be at age 35 after several years out of the NHL, and what the risk is of him retiring or fleeing once again for Russia before his contract expires, I don't think offering him a deal longer than one year makes sense, though there is likely some NHL GM out there willing to take the risks.
The risk of him bailing back to Russia or just retiring from hockey isn't so great for teams that don't usually spend to the salary cap anyway because having to account for his cap hit may still leave the team under the cap (or at least not having to make so many moves to get under it) so Columbus might be in a better position than those teams who spend to the cap on a regular basis.