Why would either team do that? I get the B's need to shed at least one defenseman, but trading him for a guy who doesn't fill your hole on 2RW isn't how they need to do it. Fischer is young, but doesn't fit the mold of a bruins player either. Below average corsi numbers, not a scorer, and an average (maybe a tad above, but not massively so) skater. he is a big body, and I guess the bruins could use him - I just don't think he's the right fit for the Bruins. On the flip side, why would the Coyotes trade away any offense, for a defensive D man, even if he does bring some snarl (which they could honestly use). The Coyotes are bottom third in scoring, and need to ADD offense, not take it away.
I don't mind trading Miller (though he's a great team guy), I just don't see Fischer being the guy the Bruins might be interested in. Now, you want to slide in Conor Garland, I'd listen. Looks to ave more offensive upside that Fischer, and is a better skater. I'd make that trade for sure. Almost certainly cost a bit more, and Arizona unlikely to do it for the same reasons I made above, but worth it if they could swing it. I think he'd do well with Krejci and Debrusk (once Bergy is healthy).
One other reason I don't see either of these trades happening is that the Coyotes are playing winning hockey, and Chayka seems intent on only doing deals that have a solid impact this season. I don't think moving either of those forwards does that for their team. plus, the Cototes are still finishing rebuilding, and trading a young cost-controlled guy for an older pending UFA isn't good asset management on their part.