I'm always amazed at how panicked some of our fanbase get during transfer windows. Just because we're not spunking cash on big names (yet) doesn't mean it's a shit window.
We won the treble because we have the best manager in the world mixed with absolute winners. The rocks to that treble-winning side are still here - aside from Gundogan - and none will really leave. While I absolutely love Mahrez and would prefer to keep him, he's not a vital reason we got over the line in the league or CL. Players like Ederson, KDB, Dias, Stones, Ake, Akanji, Rodri, Haaland, Grealish are and they're all staying firmly put.
I see Mahrez going as an opportunity for more minutes for Foden out there (his best position IMO) and the chance to promote a youngster. Let's face it, Foden needs to be starting every week and his directness, trickery, pace, passing and finishing makes that RW spot his if he wants it.