If Tarasenko's trade request is still in force, and if the Blues trade for Tkachuk, I think Calgary might need/want Tarasenko as part of the return. The Blues would need it, to clear a chunk of salary. Calgary (even if Vlad won't re-sign there) would need to have some star power and more than pure futures to show for such a trade, or the offseason looks like a complete train wreck for them.
I think the Blues could offer Tarasenko, Perunovich/Mikkola/Barbashev, a 1st, and maybe Bolduc. I think Neighbors slots in this season, so they won't include him. It looks like a nice package for Calgary, but it ends up being another trade where Armstrong gathers a lot of spare change for the best piece coming back. Losing Tarasenko means little, since he's probably gone after this season anyway, and he's late enough in his career that his next contract looks unappealing if you can get a younger player at his spot.
I kind of think Tarasenko will end up on the east coast after the dust settles in a couple years.
Calgary's front office is in a tough situation. I think the Blues will pay more than we (the forum) expect to Calgary for Tkachuk. But most of it will be to clear sufficient salary, or will be futures that don't figure into the current Cup window. Calgary has no leverage, and they desperately need to do something to make this offseason not look like a complete cluster headache. I don't think they were nearly as close to being a true Cup contender as their fans thought they were, but its over now.