Why would we want a power play specialist whose underlying statistics (not just +/-, which is largely team-based, but also individual statistics) suggest he actively makes his team worse? A top four defenseman would be nice (but there's no reason to forget that we do have Sergachev and Cernak), but we don't have that much cap space. Johnson has an NTC and I doubt he waives it to go to Buffalo, and if we do manage to move for Johnson, it will be for cap relief. I don't understand the Ristolainen to Tampa rumors; he doesn't play a role that fits anywhere on our roster.
I'm sure Buffalo would accept this for Cirelli, but that would be completely awful from our side. Cirelli makes the team better in every situation and fills a role of need; Ristolainen is the exact opposite. Cirelli is on an ELC and could probably be bridged for less of a cap hit than Ristolainen would consume. Erne also probably gets this deal done (in fact, this deal is probably acceptable to Buffalo as it), but again, Erne is a physical player for whom we don't have a proven replacement. None of these moves make any sense.
The expected goals for and Corsi for both decline with Ristolainen on the ice. Though the right graph shows his power play usage, we don't need him there (our power play is far from the greatest of our problems) and he's not better than Hedman or Sergachev on the power play. There's an interesting analysis about this
here. Overall, any trade for Ristolainen is probably a bad deal; he doesn't fulfill a useful role for us and we can't afford him anyway.