The current law is that things in the original work are copyrighted from the time they are published, and additions or updates to the character start from when they are first published. If I draw a character today, the copyright clock starts today. If in 10 years I draw the same character but with a silly hat, then the clock for the character and hat starts from that date. It’s a little weird in the modern system, since both end up being the same (if since it’s author’s death+70 years), but in prior systems that have the clock run from publication, that’s how it works.
And these prior systems still matter, since there are works like Sherlock Holmes where most of the character has entered the public domain, but certain revisions remain under the copyright of the Conan Doyle Estate.
To more concretely answer your question, if you release version 1.0 of your game today, with “Space Marines” fighting each other with a shooting stat of 10, then in 10 years release an update that gives +5 to the space marines shooting stat, updates the art for the marines, and adds “Firebats”, then the parts no longer under copyright would be the 1.0 binary of the game, the old version of the Space Marines art, and the stats. What would be subject to copyright would be the 1.1 binary, the new space marines art, the new shooting stats, and the firebats art and characters.
For example, much of the DnD kerfluffle is over the open rulebook and monster manual. If you make a game that has a Blue Dragon and operates totally differently from the "Adult Blue Dragon" that's in DnD you're probably OK.
But if you make a game that has an "Blue Dragon" with an Amour Points of 19, a Health of 255, a Speed of 40, but 80 while flying, and a Str 25, Dex 10, Con 23, Int 16, Wis 15, Cha 19...then you're a lot more likely to get into trouble.
So you make make a knock-off DnD that has pretty much the same _mechanics_ its true that you're probably fine. But once you start borrowing all the of stats of things from another work that fit into those same mechanics... well, I Am Not A Lawyer, but that's definitely the kind of thing that you'd want to start running past a lawyer.
