To come back to your mall question, I would go for something you would buy in the mall and maybe there is none:
- Phones, computers, games, book or whatever
Selling services is not really something you would look after in a mall, and which normally require sometimes. So for me not exactly a good fit for a mall business. Instead a mall business is for some kind of a quick sell kind, where showing products and giving then directly would be the way to go.

So investigate what kind of shops there is in the mall already, and then try to find something which is missing and could sell and make profit. BTW to calculate the profit you need to think about how much the rent of the mall placement will cost you per week or month or year, and see how much you can make by each sale, therefore you can see how many sales you need to do to reach a flat level:

Selling ice scream
Make .50 cents each time we sell an ice cream (like buying the ice for 1$ and selling for 1.50$)
Mall placement costs 500$ per month
that means we need to sell 1000 ice creams per month to just pay the mall placement.

See ? Now of course you should as well count all the remaining costs: marketing, salaries, electricity, cleaning etc...