What Web App should I develop?

So I've been sitting here thinking about different ways people make money online.

The limits are endless, yet I can't come up with an idea good enough that it would make me a $1 in profit.
So I decided to get on here to try to have a conversation with some of you guys regarding
what web app I can develop to generate some money.

This is how I look at it.
I could develop a web app that allows the user to use the service it features,
but with the premium features locked behind a small fee.

For instance.

RsLookup is a website that functions like a database.
It contains a lot of leaked usernames and passwords for a specific game called "RuneScape"
but I am pretty sure it required a small fee to be able to access it for a week or so.

I could easily develop something like this and it's something people would purchase no doubt about it.
The reason I'm not doing it is because I am not sure how legal / illegal it is.

And that's pretty much the end of my ideas.
But you get the point, I am trying to come up with an idea that I can have running that is free but it offers a paid version of it at the same time.
All tips are highly appreciated!