We follow the guiding line offered by canonical operators on the full Fock space, in order to identify what kind of cumulant functionals should be considered for the concept of bi-free independence introduced in the recent work of Voiculescu. By following this guiding line we arrive to consider, for a general noncommutative probability space (𝒜, φ), a family of "(ℓ, r )-cumulant functionals" which enlarges the family of free cumulant functionals of the space. In the motivating case of canonical operators on the full Fock space we find a simple formula for a relevant family of (ℓ, r )-cumulants of a (2d)-tuple (A1,…,Ad, B1,…,Bd), with A1,…,Ad canonical operators on the left and B1,…,Bd canonical operators on the right. This extends a known one-sided formula for free cumulants of A1,…,Ad, which establishes a basic operator model for the R-transform of free probability.