Japan RWC will be amazing whether you do it alone or with someone - depends what you want to do really. You can do it alone and you will have an excellent time in this completely different culture and on match days enjoy the party atmosphere at the rugby - the Japanese people are more than welcoming and will ensure that you enjoy their country. As someone who works as a Japan travel specialist, doing it with someone who knows all about the place, where to go and how to get there, it will allow you to make the most out of your time in Japan - See the rugby, enjoy the atmosphere, see some of the 'must see' sights/hidden gems, insight into the culture and turn a great RWC into an amazing one. Japan is going to be like no other RWC and a must for any rugby fan looking for something different. NZ RWC was good, England was great (apart from the England team), Japan will be out of this world.
Tips for doing it alone - Book a Rail pass, stay at least one night in a traditional ryokan (a Japan only experience), book your accommodation before you go to Japan, learn a few words of Japanese ('Ganbatte Nippon' - when supporting Japan
- it will make you friends straight away), learn how to spot the words, 'nomihodai' in Japanese - drink as much as you like - usually for about £20 and for 2hrs, take an 'onsen' ho spring bath for a bit of post match relaxation....there are loads of things as well as buying your tickets and flights, but it will be great.
And Japan is cheaper than you think.
Happy to offer advice if needed, or take a look a look at InsideJapan Tours for ideas. Cheers