Your course of action really depends on how much work and effort you can put into the expired domain. If you're barely able to maintain and optimize your current site, you probably want to just 301 redirect the old site (note: see my amended comment above about the link value not likely to be passed). If, however, you're more creative and have some time on your hands, you can try your hand at crafting a microsite. If you really know your stuff and are experienced at making money off various websites, you'd probably do well with the third option.
While getting a new domain name will give your site a clean slate and allow you to start a fresh marketing campaign, you may not be aware of the fact that some expired domains come with a high Page Rank value. A company that buys such a domain will not have to work so hard on website promotion and backlink building techniques. Thanks for updating this really informative post!
The second option requires a bit more time and effort than a 301 redirect. You could do a mini overhaul of the site and turn it into a microsite for your main domain. This option is good for exact-match domains for your targeted keyword, and there are other reasons for going the microsite route that Rand's highlighted in his post about root domains, subdomains, subfolders and microsites. This strategy also works better if the old domain has decent rankings for the keywords you're targeting.
I want to purchase a domain name that is to expire very soon. After looking it up on Whois, the owner is revealed to be 1 and 1, which doesn’t have a designated auction house based on your chart above. How should I go about trying to buy this? Because I don’t know if it will go to auction, and if it does, which auction house I should look at. It is set to expire soon: 2015-09-21

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