"what's in store" vs. "what's in stall"

There is no usage of in stall for you and it is not likely to be a common error for English speakers.

A stall is:

an individual compartment for an animal in a stable or barn, enclosed on three sides/small space; synonym: stable

or

a booth or stand in which merchandise is displayed for sale

It may be an attempt at a pun, using stall for store (a place where merchandise is offered for sale), or an outright mistake by someone misunderstanding the language.

For a restaurant to refer to a stall (a place to feed animals) would be found (mildly, at least) offensive to many people. However, as a mistake, it's a good one.

Stall and store are not related. It is simply an error as far as I know. It is not similar enough, imo, to be an eggcorn:

an idiosyncratic substitution of a word or phrase for a word or words that sound similar or identical in the speaker's dialect The new phrase introduces a meaning that is different from the original, but plausible in the same context, such as "old-timers' disease" for "Alzheimer's disease".

Baited and bated are homophones, and homophone substitution is easy to do with unfamiliar words (such as bated).

Geoffrey Taylor humorously (and consciously) captured this misuse in verse in his poem Cruel Clever Cat (where Sally is the cat):

Sally, having swallowed cheese,
Directs down holes the scented breeze,
Enticing thus with baited breath
Nice mice to an untimely death.