The shopping centre that would get loads of visitors but without the Westfield price tag is the one by IKEA in Hammersmith (though I guess that’s Fulham territory).
That said, I really think the old Kew Bridge distribution centre has loads of potential for the pre‑game experience. Since Mercedes built their fancy new showroom across the road, that old site’s barely being used. Brentford could snap it up cheap and do something proper with it.
Imagine a big car park there (matchday parking would pay for itself in a couple of years), and then on top, a proper club shop, maybe a few bars and restaurants for before and after the game. It would be buzzing on matchdays, and it’d bring in decent money for the club too.