I still can't believe the Islanders didn't trade him at the deadline when he still not signed to an extension at that point. This isn't just hindsight, I said it at the time, it just seems really irresponsible.
I can't see his rights being worth a lot, unless they are going to go outside of the box and try to work out a sign and trade for an 8 year contract. If I recall, that is something that hasn't happened yet in the cap era, so I am not sure how plausible it is.
Something like a 2nd or 3rd round pick that becomes a 1st if Tavares signs might be fair for his rights if there is no 8 year sign and trade involved and it is purely a deal for Tavares rights. I can't see why Tavares would sign anywhere before July 1st though, so it really doesn't make much sense for a team to give up major assets.
The only way I can see trading significant assets for Tavares rights in the blind hope that he signs working would be if a team is going to overpay and offer him something like a max AAV (16M per season x 7 years). A type of take it or leave it offer that Tavares has to sign before July 1st, and that might not be there afterwards. Tavares isn't a 2nd/3rd liner like Soderberg who had his rights traded, was offered a solid deal by COL, and was smart enough to take it while it was on the table instead of risking COL moving on and other offers not being as good after July 1st. He is a superstar who knows that almost every team will line up to try and sign him. So if a team gives up assets for his rights, they have to be prepared to go all in and offer him something ridiculous.