"I bought a 135sqm office space on Great Portland Street. What sort of business should I build on it?" is almost as specific question as the above
On a more serious note, I suggest you to look into several ideas/methods discussed on this forum and choose to go with the one that you think you would be the most successful with. Nobody is going to hand you a business plan on a silver platter.
As for the domain - it's so generic that it's very hard to suggest any specific niches for it to be used.