As others said you will be giving up the best player in such a deal.

__________________
8 team keeper league/roto. Keep 12+2 under 200 games played.
G, A, PPP, PIM, FW, SHP, GWG, HITS, SOG, BLS W, SV, SV%

Lw: E.Kane, Benn, Moulson, Neal

C : Plecanec, Tavares, Stamkos, Gagner

Rw: Voracek, Backes, Stewart, Landeskog

D: Burns, Seabrook, Subban, OEL, Voynov, Edler

G: Quick, Crawford, Anderson

Bn:Eriksson, Steen, Skinner
ir: Kesler

1. Backstrom
2. Thornton
...
3. Couture

I doubt I'd trade away Karlsson.
Karlsson's BEST fantasy hockey value is in points-only leagues. The difference of Karlsson above a waiver-wire defenseman is possibly 35pts. (Karlsson is ~65pt player, WWD will probably be around 30pts.)

A waiver wire forward in most leagues is about 50pts.
So getting one 70-80pt forward is barely even.
If you can get two 70-80pt forwards... then yes, you take that.