I think if we signed Lemar and kept Sanchez we would have a very good and deep squad, even after selling the likes of Walcott, Lucas, Gibbs, and a few more. A title challenge would likely be undone by Wenger and his tactics in some way but I would back us to finish 2nd-4th and to compete seriously for the Europa League.
CF: Laca (Giroud)
AM: Sanchez, Ozil (Lemar, Welbz, Iwobi)
LWB: Kolasinac (Monreal)
CM: Ramsey, Xhaka (Wilshere, Elneny, Coq, Cazorla maybe)
RWB: Bellerin (Ox)
CB: Kos, Mustafi Holding (Chambers, Mertesacker, Gabriel)
GK: Cech (Martinez)
Ideally, we'd add another more reliable top class CM to this team but there really aren't many holes. Lemar could also play the LWB and perhaps CM roles in some matches so I think he'd be a starter more often than not and would be a key contributor.
The problem is really selling Alexis and keeping Walcott or Lucas, which I suspect is Wenger's plan. Do that and all of a sudden this team lacks true quality up front and likely struggles for goals.