So let's just say that this year and next matches his last 2 years (20+ goals and 60+ points). I know he's pacing above that right now but whatever. He'll be 28 and a half when the first season of his new deal kicks in.
Trying to find a somewhat recent comparable, maybe take Couture and adjust down for positional importance and playoff production (so far).
Couture signed $8 x 8 for ~10% of the cap. Cap might be ~$82M when his time comes around, so $8.2M, knock about a million off and settle in $7 - $7.5 range on a long-term deal. Maybe not the full 8 years, but in the 6-8 range.