Expired Domain Advice

As some of you will know I have been having gugol issues with one of my sites. I have tried to work out what is wrong but haven't come to a definitive conclusion. It's possible however that it may have something to do with expired domains.

I am looking for some advice, what would you do with these expired domains.

I purchased about 3 or 4 off Godaddy, the domains had a lot of backlinks from adult sites and I reckoned they would bring in some decent traffic.
I then redirected the domains to my main site and almost immediately got penalized by Google. I decided it must be due to the huge number of backlinks that I had suddenly picked up (900,000) pf them on a pretty new site.

I disavowed the sites and bought a domain. I then redirected the traffic to these with a link through to my sites. This seems like a huge waste.

I also used to own a forum that over the years picked up many spam threads. I deleted the site and redirected that site to my main site as well. Unfortunately the spammers had been spamming that forum all over the web as well so I picked up more bad links. Again I removed the redirect.

Thing is, if you bought a domain with good traffic what would you do with it? I don't really want to build a site on it although I guess I could do a very basic one.