1. Wait for copyright to expire.
Life of author+70 years, or 95 years from publication, for works authored by a company (and not a group of individuals). Frozen, from Disney, falls under the latter category.
Even so, you might run into trademark troubles. Mickey Mouse (the Steamboat Willy version) is public domain but Mickey Mouse is a trademark of Disney in around 70+ categories. Their lawyers can come after you for that.
2. Make the fan art/fiction/game but never distribute it (until #1 comes into effect, at least).
You can make your fan game and it is entirely legal for you to do so. Unless you distribute copies (including a full transfer the single copy you made), you do not violate copyright.
There are some who say you can distribute fan art and it's legal so long you do not profit from it. Doing so is not "entirely legal" as the question demands.
Some companies, like Games Workshop, went after fan-art/fiction/videos and enforced their copyright to the point of suffering community backlash.
There is a fear in these big companies that if they don't enforce copyright thoroughly, courts might see this as a dilution of their power to do so.
3. Make the product but enact changes to go around to skirt around the copyright (by a wide berth).
"Fifty Shades of Gray" was originally a "Twilight" fanfiction. The author then remove all references and allusions to the vampire novels, and marketed it as an original work.
Citing a recent example, Palworld is very similar to Pokemon. But it doesn't fail the informal "squint test"
In layman’s terms, a good wayto tell if a copy should be allowed is to ask whetherit fails the ‘squint test’: If you need to squint to seethe difference between two designs, then one is aninfringing copy of the other.”[ref].
Game mechanics cannot be copyrighted. Only music, graphics, character design, text, lore, and so on.
You can make a game that plays similar to pokemon. Back to Palworld, one thing pundits pointed that made the games different was the violence.
Unlike Pokémon, Palworld is a more violent game where players can wield a variety of weapons like machine guns, rocket launchers and spears. Its gameplay includes brutal options like being able to butcher pals and using pals as live shields in combat.[ref]
Since this is recent news (at the time of this posting), it still remains to be seen if some legal action will be attempted by the holders of the Pokemon franchise.
4. Buy a license.
Easier said and paid than done. The copyright owner might outright refuse.
5. You might get sued/receive a DMCA takedown, get Copyright Struck, nonetheless. Even if you did #4.
Unless you bought a license, there's nothing stopping the (assumed) bigger company from coming after you. They might file a lawsuit that will end in a nothingburger and force you to spend legal fees to defend yourself.
At least, expect a C&D letter. Sending a threatening letter costs next to nothing to them. (disclosure: I received a couple in my life). The C&D letter might be just empty threats.
Copyright bullying is the younger sister to Trademark and Patent bullying.
In one case, a YouTube author got a copyright strike from a song they licensed from the OG author after said song ended up in a big label portfolio. Or even with original song, due to alleged algorithm mistakes.