Vegas is at a cross roads with Karlsson.
They could opt for a 1 or 2 year show me deal since his level of production is completely out of line with his previous career totals, but if he continues to be a near point per game 30+ goal (yeah I know he has 40+ now) 1st line C, they are going to end up having to pay him something like 8-10 million over 8 years on his next contract.
If they go long term right now, they'll likely get a bit of a discount due to his limited body of work as a 1st liner, but if he regresses, he won't be able to live up to the contract.
I would probably predict something like 2 years 5.5-6.5 per. That buys out the final two RFA years of Karlsson's current contract, gives him his first big pay day (11-13 million) and leaves the door open for a big 8 year extension 1 year from now if Karlsson has a strong 2nd season.
It doesn't really matter all that much what they pay him on a short term show me deal, because if he regresses, the deal will quickly be over with, and the Knights have a surplus of cap space. If they are confident in Karlsson, and he is willing to go the safe route, they should lock him up 6-8 years now.
The closest comparable situation I remember to a player breaking out like this (although different positions) was Klingberg on Dallas. Klingberg took a discount for the security of a long term contract after 1 very strong season rather than better on himself with a show me type contract.