10m AAV on an 8 year deal is probably a good deal for both sides when you take into account that half of those years are RFA years. 7.5m for the RFAs and 12.5 for the UFAs.
12.5M would be on the low end for his UFA years. Look at what Panarin just got as a UFA this year with slightly worse stats then Mikko. Now factor that Mikko's UFA years would be 4 years down the road with a cap that will be much higher.
In order to get Mikko on an 8 year deal the contract would have to be structured something like this:
Year 1: 7M
Year 2: 7M
Year 3: 8M
Year 4: 10M(Technically this is an RFA year but as we've seen with the bridge deals recently, its being treated as a UFA year by players and teams essentially)
Year 5: 12M
Year 6: 13M
Year 7: 15M
Year 8: 15M
In order for him to actually sign and 8 year deal you have to really make it worth it in those final years. Because by signing an 8 year deal that means he becomes a UFA basically at age 31 instead of age 28 or 29(Like he would on a 5 or 6 year deal). And as we're seeing more and more now, teams are spending money younger instead of on older guys. At 28/29 he could probably get another 7 year deal in the 11-12M neighborhood. But at 31 it would be much harder for him to get that 7 year deal and at that kind of cap hit.
So again, I really think to get him on 7 or 8 years you have to pay a lot for those last 2 years. Which is why a 5 or 6 year deal(And for the Avs really a 6) is what makes the most sense for both sides. Avs get to buy 2 years of UFA while likely still keeping the cap hit around 9.5M.
A 6 year deal structured like the above would carry a 9.416M cap hit. While adding those extra 2 years would bump it all the way up to 10.8M. But that is what it would take to get him for 8 years IMO. I would actually still quite easily prefer the 8 year deal at a 10.8M cap hit because 10.8M in 7 years from now from a likely PPG+ winger is very likely to be a fantastic deal.
But I suspect Mikko and the Avs settle for 5 or 6 years in the neighborhood of 9.5M.