I have found a number of instances where I have wanted to generate a 
variant with varying degrees of branching, e.g.

\cs_generate_variant:Nn \tl_case:Nn { No }
\cs_generate_variant:Nn \tl_case:NnT { No }
\cs_generate_variant:Nn \tl_case:NnF { No }

I find myself envious of the functionality available with 
\prg_new_conditional:Npnn and its ilk where multiple degrees of 
branching can be coded in a single statement using a subset of { p, F, 
T, TF }. It would be nice to be able to write something similar, e.g.

\cs_generate_variant:Nnn \tl_case:Nn { No } { , F, T }

where the empty slot before the first comma denotes the nonbranching 

I don't imagine I'm the first to have thought this, so presumably there 
is good (or at least some) reason for not providing the functionality. 
It would be good to know.