If you want to create a business that provide useful tools for portofolio management, or whatever my suggestion is only to do a market survey. Keep in mind that customer is everything so that what you call useful could be useless for others.

I have ever seen a website that have price comparison between exchange but I can't remember the name. Personally, I don't need it cause I can use manual check and sometimes I can't really believe in a platform (system errors do happen). However it could be useful if the performance is optimal.