That’s quite complicated and gets healthily outside anything I can even feign strong knowledge on. I was involved once with a relative in just getting the filings done for securing some trademarks related to a private book imprint and suffice to say this isn’t an area of the law friendly to people trying to figure it out on their own.
There’s a bit of dissension here. In short, most people agree that as a “practical matter” you can trademark a character, and that makes it incredibly difficult for anyone to produce a derivative work of that character, even one that would not violate any held copyrights. However, many people who believe that also believe that goes against the “intent” and text of the copyright law, and that this status quo is essentially the product of basically Disney and a few other big right’s holders being so scary for a small artist to litigate with no one has really pushed the envelope very far, plus a few somewhat industry-tilted court cases.
But what you could do is once the early Disney library enters public domain, you could start selling DVDs (if people still bought DVDs) that literally contain those videos–just like the works of Charles Dickens can be published by anyone who wants to publish them. Note you have to be careful though–you can’t just grab a Penguin edition Dickens book published in 1988 and copy paste it and sell it, that edition enjoys copyright for some years after publication. You would have to quite carefully to go back to a version of Dickens’ work that you know is definitely in the public domain and reprint that, not anything containing a modern edition’s text. [Interestingly Dickens’ works never enjoyed copyright protection in the United States even during his lifetime, but that’s another story.]
However
But what you could do is once the early Disney library enters public domain, you could start selling DVDs (if people still bought DVDs) that literally contain those videos
While that ought be true, it may actually not be. Interestingly despite all the drama about Steamboat Willy and other early Disney titles potentially hitting public domain, there’s significant evidence that Disney did not properly register copyright on any of those early works at the time, and that under copyright law they actually are in the public domain right now. Someone could already be doing their own Steamboat Willy DVDs and selling them. Why aren’t they? For one, because no one wants to test that legal theory in a court against Disney. And for two, even if they were successful in arguing Steamboat Willy was in the public domain, they don’t want to get sued by Disney for trademark infringement. Disney would argue that you are confusing customers and making them think they are buying a product from Disney.
Now, there is firm precedent–you cannot use trademark to extend an expired copyright. But there is enough of a case that it would have to be litigated. If you’re a fly by night DVD imprinter you simply cannot meaningfully litigate against Disney. If you go 5 years and win, you have the ability sell DVDs that probably won’t gross you more than $100,000, and you are likely bankrupted by legal fees.