Again, I'm not Doug Armstrong, and I don't speak for Blues fans everywhere--but if I'm the Blues' GM, any trade for Shattenkirk has to address a team need. There would be lots of disagreement over how to prioritize those needs, but the way I see it, the Blues most desperately need:
-A #1A/#1B center
-A tough power forward to replace the physical presence of Backes/Brouwer
-Cap friendly contracts
As I said earlier, I absolutely believe Shattenkirk would gladly sign a new contract to play and stay in New York.
McDonagh is an enticing player--any team would want him--but the Blues' would have a hard time fitting him into their cap structure. If we could get out from Bouwmeester's contract, that would be a different story. As currently constructed, the Blues wouldn't want McDonagh as part of any trade. [edit: Blues are looking to move salary from defense and put it into upgrades on offense; long term, trading for McDonogh doesn't help them do that.]
Brassard would be a great addition to the Blues, but again, his salary would be problematic, and I don't think the NYR would move him anyway.
The only pieces I see that the Blues would want from NYR are Kreider, Stepan and Miller. Kreider straight up would be my preference, but it seems that most on here think I'm undervaluing Shattenkirk in that deal.
Someone else suggested a package around Shattenkirk and Schwartz for Stepan and Kreider. Schwartz is by far the best player mentioned, so that conversation would be pretty short, unless you were adding Miller--which tips the scales too far in the other direction. And I can't see NYR giving up that much youth--those players all project as core parts of the Rangers' future, and that's too many moving parts.
I suppose it's too ambitious to dream up a solid hockey trade for both teams, but that's what I'm looking for. Help me out.