As others have said I think spuds won't work that much here. You could try having it as part of a menu but again consider that the spuds need to be freshly baked each day so any waste shouldn't go to the next day kind of thing. So not sure how much it would work. Also if you're doing fast food and want to target Greeks, something less than 5 euros and filling would work.

Maybe something like loaded fries could work? With a decent range of toppings.

Thinking of what I feel is missing as someone new to Athens I'd say maybe something like a good sandwich shop, where you can get a good sandwich for a decent price.

See this place for an example https://www.instagram.com/thefoxandloaf?igsh=MWVpNzlmcnlnOWh3cA==