I still don't understand why he is untouchable. You have fox who is a top RHD in the league and Trouba who is probably playing the best hockey of his career and is under contract for a while. The right side is full in NY. To get a good player it is going to be quality over quantity.
Rangers will probably end up moving Trouba when his trade protection ends (even though he's having a good season) and view Schneider as his replacement (it's no coincidence that Schneider's contract is up when Trouba's protection ends in 24-25).
If they want a top 10-15 center in the league when he is on his game, it is going to be quality, not quantity.
Scheifele isn't a top 10-15 center in the league rn. His defensive play is horrid. You could easily argue that McDavid, Matthews, Draisaitl, MacKinnon, Bergeron, Crosby, Barkov, Stamkos, Point, Tavares, Kopitar, Aho, Lindholm, O'Reilly, Zibanejad, Eichel and Malkin are have a better impact than him given their all around performance (some of these being elite defensively while maintaining a good offensive game). You could also players like Kadri, JT Miller, Larkin are outplaying him this year as well. I actually looked up and of all the centers listed in the top 30 of points per game, Scheifele is the only one with a +/- worse than -11, and he's -21 on the season. Obviously it's not the most fair stat but it goes to prove my point about his two way game.
I think slightly less than the Eichel return would be "best case scenario" for WPG, and that is with a bidding war. Middle six forward with team control, good prospect, and a 1st.
Lundkvist is a very good prospect -- I will say that he definitely needed some adjustment time to the ice size but he didn't look lost in NHL (especially with Nemeth as his partner, choose a worse player to be hampered with) and now has turned his bad AHL start (1 point in first 9 games) around the past two weeks (10 points in last 11 games).
Middle six players are something the rangers don't really have in spades rn, but they do have good young players projected to be solid middle sixers like:
Chytil - still only 22 with over 200 nhl games played, former 1st rounder that has the ability to drive play (avg of +4.1 rel CF%). Proved last year he didn't belong in the AHL. Probably better suited to play wing than center TBH solely because his faceoff ability (his two way play is fine, perhaps if they have a winger good at faceoffs they can switch them just for faceoffs) ; and
Kravtsov - former 9th overall pick, some attitude issues, performs really well in the KHL as a 22 year old, noted playoff performer there w/ 24 points in 30 playoff games since his draft year.
They also have their first rounder this year and two second rounders in what is supposed to be a deep draft.