I would still recommend going after a brand new product category. While it may not sound "ambitious", but more like you're building a toy (say a drone or an AI

I would still recommend going after a brand new product category. While it may not sound "ambitious", but more like you're building a toy (say a drone or an AI machine), it's most likely that the next gigantic company is not a new search engine or new operating system. It's extremely difficult to get into an existing product category such as operating systems, search engines, email, etc.