The salary cap being the problem to every trade involving big assets may be resolving itself since the Penguins have recently waived two players carrying sizeable AAV.
Kasperi Kapanen was notably claimed by the Blues and today they've sent
Brock McGinn down the waiver wire. Kapanen accounted for a $3.2M AAV, while McGinn accounts for $2.75M. Which makes more than enough room to take Miller's cap hit which is $5.25M until the end of the season.
Jason Zucker's contract is expiring and would therefore leave enough room for Miller's next deal to come in.