I think that sometimes people get too hung up on winning every trade that they want to start every conversation with a low ball offer. Why not just try letting the guy know you are interested in Pietrangelo and ask him what it would take?
The other thing is I have to wonder why you would want to take a stab at Pietrangelo right now. His stock couldn't be higher. He's young, coming off a breakout season and is still on a highly affordable one-year deal. If you acquire him now, you'll be buying high (so paying an exorbitant price) on a player who could regress or at least level off but more importantly is set to receive a huge pay raise after this season, depending on what the new cap rules are by next summer.
You want to supplement your defense, that makes sense because you are really young there and he has plenty of nice pieces that are a little farther along in their development that could help you, so there is seemingly a fit whether you go after Pietrangelo, Kulikov, Hamilton or whomever.
Now personally, I'd go after Kulikov. Yes he has trouble staying healthy but he has improved every year in the league and is going to pop very soon. He's still an unsigned RFA but he hasn't performed so well that he might land a ludicrous contract plus the new CBA should restrict his salary even further. I suspect he'll land one of those short "show me" types of deals we see young players who haven't broken out yet typically receive.
Again, I'd ask the GM what he wants but I suspect offering Richards or Lindback might get the conversation rolling towards landing Kulikov. Now if you really must land Pietrangelo and you really must make an offer to get things kick started then I think Ryan straight up makes a lot of sense for both parties but I can see him scoffing if for no other reason than Ryan is "old news" and Pietrangelo is fresh.
I've also seen "Chara must go" in discussions with you landing Pietrangelo. I don't see it. This is points only with a salary cap. Chara pretty much caps out at 50 points and yet he's making over $7 million a year. I could argue that he's not even worth owning at all at that price so if Chara is the piece you "must" include to get this deal done I say go for it. Pietrangelo is a vast upgrade because at worst he's going to have a similar salary as Chara but is way younger and thus more valuable.
But from perspective Chara isn't a must have for your potential trading partner. He's deep on D so he may not need an established replacement for Pietrangelo. What he needs is forwards and maybe a goalie so again if you must make an opening offer I'd look in that direction.