This may seem like a pie-in-the-sky speculation question, but I have good reasons for asking this.
Is there any sense in which $H^{-1}(G;M)$ is defined for a group $G$ and a $G$-module $M$?
The reason I suspect this may make sense comes from some of my work with Kassabov and Vogtmann, where we consider cohomology spaces $H^{2n-3}(\mathrm{Out}(F_n);S(V^{\otimes n}))$. Here $V=H_1(\Sigma_{g,1};\mathbb Q)$ is the first homology of a surface of genus $g$ with one boundary component, $S(V^{\otimes n})$ is the symmetric algebra on $V^{\otimes n}$ and $\mathrm{Out}(F_n)$ acts on $S(V^{\otimes n})\cong S(V\otimes \mathbb Q^n)$ via the $\mathrm{GL}_n(\mathbb Z)$-action on $\mathbb Q^n$. (Taking the base field to be $\mathbb Q$.)
The reason we were interested in these spaces is that we could embed the abelianization of the Lie algebra $\mathfrak h^+_V$ which is the target of the Johnson homomorphism on $\mathrm{Mod}(g,1)$ into the direct sum of these cohomology groups. Well, almost. We had a separate discussion for the $n=1$ case, where you would be looking at $H^{-1}(\mathrm{Out}(F_1);S(V))=H^{-1}(\mathbb Z_2;S(V))$. Here $\mathbb Z_2$ acts on the symmetric algebra $S(V)$ by $(-1)^{k}$ on $S^k(V)$, and the answer "should be" $\oplus_{k=1}^\infty S^{2k+1}(V)$, which in some sense is the anti-invariant part of $S(V)$. So one hypothesis might be that $H^{-1}(G;M)= M/M^G$, but I don't see how to justify that.
