bxl-forever
Most likely Queen Victoria, who died in 1901 and is related to the Belgian Royal family, whereas this street was created or named around 1907 when this part of the territory became part of Brussels-City.

But we have no certainty about this, it’s only an educated guess.

Filling etymologies of all streets within the Brussels-Capital Region was part of the EqualStreetNames project a few months ago.  If no-one could find the right source then, there is little hope we can beat the experts.

User @ebruxelles, who maintains a personal website with such information  has already looked for this item too and apparently could not determine the source reliably.
https://www.ebru.be/Streets/streets1000/bruxelles-1000e-avenue-victoria.html

I wouldn’t mind if you add the etymology for Queen Victoria (Q116728), we can always change it later if someone finds the "real" Victoria.  Or you can alternatively leave it empty.