The ending -an has been preserved in a number of expressions, almost all that I can think of are adverbial in nature (meaning, the -an doesn't show say the direct object of a verb, like it would in fus7a). In Syrian for example, you can choose between "more colloquial" and "more fus7a" alternatives, like a7yaanan vs. marraat, or mubaasharatan vs. dughri. Shukran is just another example of where this has been preserved. The -an in shukran as used in colloquial shouldn't be considered "grammatical" as manSuub, as it would be in fus7a. It's just a frozen part of the word. However, when colloquial is written, there's obviously a preference to write it شكرًا rather than something strange like *شكرن.
Also there are a number of dialects that preserve tanwiin to mark indefiniteness (though they don't preserve the distinction between the final vowel itself, I think it's usually just -in).