I never really answered the question. I would want first and foremost to sell healthy stock, not matter what the species. Beyond that I am not sure how I would want to go. I see most stores as falling into a few basic categories. The first decision is would the store sell FW, SW or both. Since I have never kept SW fish, I would probably go FW only but then I would also want to include live plants.
The next decision I would need to make would be an assessment of the nature of the customers in the area. Are they more budget conscious or more affluent. Are they likely to be on the less sophisticated side or are there a lot of fish nuts. One way I would want to keep a lot of the less expensive species but also a few of the rarer fish which would be a bit more expensive. These factors would also dictate the sort of hard goods and tank sizes I would want to stock.
Finally, how everything would work would depend on my budget. It is hard to manage inventory without considering financing. Basically, a fish store is a business and one needs to make money for things to work out OK. But if money were not a consideration I would want to have a huge store with as many different fish as possible. I would want to have the greatest assortment of equipment and medications.
Perhaps as important would be having an online operation where I would sell and ship anything I sold in the store.
I do not think I would concentrate on any species just because it interested me. What would be more important would be what I would not sell. First and foremost that would be glo-fish. Next would be monster fish. There are two main reasons for this. the first is they could only be stocked when young and small since big fish need big space and cost more to maintain as stock. This leads to the second reason . Not stocking such small sized big fish could never lead to their being bought by folks who have ni clue and might buy something they rally should not. One cannot sell what one does not have
I have worked with some rare fish over the years. I have had them breed and been able to sell them to other hobbyists. The one thing I have never done is sold them to stores. these fish should go to people who understand they are rare fish, may even be endangered and that keeping them should be done with the intention of trying to breed them.
I have softer water and in the neutral pH range. So I have never kept rift lake cichlids. I know very little about them. But If I had a store I would want to include them. I have only ever wanted to keep two angelfish- double dark blacks- a man maid strain and Altum Angels, a wild fish. I have kept both. But I would most certainly want to have a greater variety of angels in "my store."
I also would hole to have a staff that was well educated in a variety of areas. I would want to be able to refuse to sell things to people clearly not able to know how to care for them properly. I would want to offer advice on cycling for those who needed it. I would want to prevent people from buying fish they may not be able to provide for properly. I would want to be able to help customers learn and to improve their skills when possible. The best way to insure fish are treated properly is to insure their keepers know how to do this.
None of the above is really concerned so much with what I would species I would like to sell as much as it is concerned with how I would prefer to do it. The above attitude can be applied to anything for m the smallest store which has the most basic species to a a super store which offers it all.