For me, I will say the most important is not what kind site/application I will build if I have this amount of money for investment. No matter which kind of investment you embark, the first thing is "don't put all eggs in one basket", so don't use the full 100k to develop only one system.
The best I think if really emphasize on earning profit is to tender project from other company, and you are paid with a contractual term.Therefore, you don't have the risk of building totally new system with new idea which is not proven to have market share, and you don't have to face the competition on selling your product on the market. The only competition is to tender project with others competitor, and the best way is if you have good network where you can get project from, then you don't even need to face the competitor.
Why just a site/app. ? I can think of numerous other ideas too. Also, even if your product becomes popular, prior estimation of ROI is difficult. For eg, twitter became popular but foursquare & other services by Jack Dorsey didn't achieve that much response.
It was the best of times. It was the worst of times. It was a tiny ad.
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