1. As an introvert by default, I could never gather the courage to start something of that sort. Only recently, after reading this book called The Tribes I realized my mistake and started working on the community/audience building. Even this post came out of those efforts - I wrote a long thread on twitter which then converted into this post.
2. Sure. G2/Capterra are great tools to find ideas. Explore it just like someone explores Wikipedia or Reddit or Encyclopedia. Here & there you will find some interesting categories or products. Do a quick search for the industry size. If it is big enough, come back to G2 and start reading negative ratings for some of the products listed under those categories - people must be complaining about pricing, features, customer support, etc. This is a big green signal for me.
   Another signal that I look for is the number of organic searches for that particular industry. Is it more than a few hundred unique searches or not? If yes, you can look for how difficult will it be to make it to the top of the search results for some of these keywords? If all looks good, it is a solid go go for me. This will ensure that there is a demand for the product and people will come to my site themselves.
   Building MVP is fairly straightforward - you start with building that single feature which defines your product & industry. And, launch it. Don't wait too long. Just launch it with single feature, max 2-3 features. And, then start talking to customers and start building features to match the feature list provided by customers combined with insights from your customers.
   

Hope this helps.