Almost immediately after that post went live, German publisher Hans im Glück released solitaire rules for Carcassonne in German and English that you can download from its retail site Cundco.de. (Note: The product is listed as unavailable, but that's because you cannot purchase it. Scroll down to the "Available downloads:" section to find download links there.)
These rules are for use only with the base game, although the publisher encourages you to try whatever combinations of expansions you want, and it can't stop you from doing so anyway. The gist of the solitaire rules is that you'll play with three colors of meeples, with four meeples of each color. Divide the tiles into three piles, and place one pile by each color. You play for each color in turn, and if you can't place a meeple on a turn, the game ends immediately. Features can score for multiple colors if the color with the fewest points is one of the scoring colors, and at the end of the game your score is equal to the lowest-scoring color.
• Working along similar lines, AEG has released rules for "Space Base at Home", a way to play John D. Clair's engine- and tableau-building game with any number of people who own a copy of the game.
In brief, one player is designated as the host and takes the role of both the first player and the third player, while everyone else takes the role of second player. The gist of the set-up, as far as I understand it, seems to be that the host is indeed hosting and not actually playing the game, but instead providing an AI of sorts for everyone else to play against.
• The roll-and-write game T-Rex's Holiday from Yu Wang and Blue Magpie Games was a SPIEL '19 release, and now the publisher has uploaded the game sheet on BGG, which means that you can download both the rules and this sheet, grab three dice, then start playing.
The twist of this roll-and-write is that the active player rolls three dice, reveals two of them for all players to use, then reveals the one die they hid, which players may or may not be able to use depending on what they thought the active player had rolled.
• Days of Wonder has released a print-and-play version of Ticket to Ride titled Ticket to Ride: Stay at Home, with players now trying to complete "routes" between different parts of the house. You need one of the Ticket to Ride base games for train pieces and cards, then you can visit the Asmodee website to download the components in A4 or U.S. letter format in up to thirteen languages.