I've taken a couple of vacations by myself that I really enjoyed (a trip to Barcelona, skiing/Sundance Film Festival in Utah, a road trip through the southeast of the United States). I'm inspired by the fact that the /r/solotravel subreddit has over 800k subscribers and is growing by around 20k subscribers per month! If you search Instagram for #solotravel you get over 5.5 million results. There tons of articles, blogs, and books about solo travel. Travel is a gigantic industry with room for a lot of players.
Looking through the posts on /r/solotravel, they seem to fall into a few main categories: seeking advice, looking for ways to work while traveling, and looking for people to hang out with while traveling.
I'm seriously considering building a site/app that's helps solo travelers find other solo travelers to hang out with, has a forum, and aggregates content (articles, blog posts, social media posts). Basically everything solo travelers might want.
There are a lot of possibilities, but initially I think a web site and mobile app with features to help solo travelers find other solo travelers to hang out with, and content aggregation from around the web to drive traffic to the site/app would be the top priorities.
As far as monetizing it, maybe use a Pay-What-You-Want model and show travel-related ads to users who are using the site for free?
I'm just about done with a landing page for gathering email addresses for people interested in it. Should have that up this weekend!
Edit: The landing page is up! https://solotravelbunch.com