It's one of my favorite buildings in Portland, I hope they do something creative with it. Ruth's Chris has done a good job of using some nice historic buildings (Boston's old City Hall for example... it has far more character than the Ruth's Chris in Providence which is in a modern high rise). Still, I don't know if I love Ruth's Chris there. I also don't like the visitor center idea... I feel like the building is too prominent for that type of use. I know you want something to get a visitor's eye, but it can be put to so much better use.
I would think it would be a nice place for year-round food stands/stalls. Similar I guess to what's in Quincy Market, but a bit smaller and perhaps with emphasis on more local cuisines and ingredients. Think LA or SF style food trucks, but with a permanent home. It has access from both Commercial and Fore Streets which would keep a lot of foot traffic coming. I'm not sure what the interior of the building looks like, but I assume the first floor is fairly open given the type of structure it is. Obviously that's important. You could put seating on the second floor and there's definitely enough sidewalk space on both sides for seasonal outdoor seating. It would be great if they could allow beer and wine to be served as well given how many local brewers there are.
I had a lot of friends in Portland that were somehow involved in the restaurant industry (sous chef, pastry chef, etc). Most of them couldn't afford a restaurant space (or couldn't organize it), so permanent stands/stalls would allow people like that an opportunity to get a start for far less in terms of overhead and startup costs (and staffing). It would be a good way for talented people to get started and a good way to give them instant exposure.