Augmenting built in mappings

I'm trying to augment built in key (adding behavior) and came up with this approach, which in initial tests seems to work:

" ======================
" psuedo code for 'smart' augmenter
function Smart()
if [some test]
[do augmented]
else
return [mapped letter]
endif
endfunction

" and then a remap call
nnoremap <expr> <nowait> [mapped letter] Smart()

" ============

Is this a reasonable approach?
Are there problems I should be aware of?

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