Tokyo and other Japanese cities had malls and humongous shopping complexes underground since the seventies - mainly because of space expense in Japanese cities.
Omotesando Hills was built in 2005 in central Tokyo, and has over 130 shops and even 38 apartments. The complex itself has six stories above ground and six stories underground.
Even small cities like Nagoya takes shopping underground, with subterranean networks of tunnels full of shops.
This could be a great model for Dhaka provided air-circulation and water drainage issues can be tackled.