A sensible plan shouldn't mention both "100000 simultaneous visitors" and "I'll have a bit of free time in the next couple months to just sketch out ideas".

100000 simultaneous visitors are a huge number that implies both progressive growth over a long time, without wasting resources in premature, oversized and overcomplicated infrastructure, and a solid business plan (in fact, an exceptional one) behind that growth, making products and services more important and more sophisticated than generic web application scalability.

On the other hand, two months of experimentation can allow you to learn a lot about highly scalable web application architectures and related technology, but building a real one requires an actual business; the most you can do on your own is reimplementing some benchmark.