There's a special mindset that is needed for a video game IP to be able to branch out successfully into spin-offs, and unfortunately some of the best video game IP holders don't seem to have it, so I'm gonna lend them mine.
The primary thing major studios seem to lack is a willingness to fail; you need to be willing to fail. Not all spinoffs are successful, and that's okay. It's hard to gauge the overlap between your IP's core fanbase and fans of the genre you're trying to get it into. And sometimes, that means you have to admit defeat.
One of the reasons this ends up being such a pain for many modern game dev companies is because they generally produce live service games, so when they admit the game is a failure, the minority of fans that got into the game suddenly have it removed, so the live service resources could be reallocated elsewhere. This is why I think building an offline-only, or at least self-hosted, version of such games is a good idea, because then you can keep it up on marketplaces to hopefully make back your investment over time, and the people who do enjoy it can keep enjoying it. Live service games don't allow for this, and although many are profitable, many, many more are not. At this point, I think major developers need to swallow the bitter pill that live service games are not going to continue to be an endless well of profit, and reconsider building games that might actually have staying power beyond a few years instead. And yes, I am aware that there are live service games that have been profitable for decades; it's still a limited market, no one can get majority market share, and there's a bottom to that well, especially when the economy is getting more and more difficult to work in and therefore people want things they can pay for once and keep enjoying as opposed to things they need to regularly pay more for in order to continue using them.
Another issue is that game development is an investment, and when you develop something truly new, that's a higher risk investment than developing another in a series that always sells millions of copies on release day. I completely understand that, I do, but the best games, the ones that are enjoyed for decades, touch people's lives in ways that matter, or keep people coming back over and over and over again were not games that were safe investments. Doom (as in the original 90's game, not the equally excellent more recent games) is still incredibly loved, but the idea of an FPS was completely foreign at the time. Undertale touched so many lives because of its complex storytelling, lovable characters, and interesting game mechanics. Stardew Valley was a single indie dev deciding to make the game they wanted to play, in a genre that was so heavily dominated by AAA studios that similar games from indie developers never took off. I can keep going, but you get the point.
So, in hopes of inspiring some major devs, let me share a few ideas for spinoffs of beloved gaming IPs that I think would go over well. Just as an example, I'm going to use Bethesda Softworks IPs for this.
The Elder Scrolls:
Honestly, an offline mobile game similar to blades would probably be great, though it wouldn't sell a lot of copies. If you make a viable port to PC as well, it could make some money, but I would recommend using the coding and assets from blades as a jumping off point to save yourself some development time, and thus some costs. The actual mechanics were good, it was the fact that it could only be played online and required either continued monetary investment from the player and/or arbitrary waiting periods. I'd gladly pay $20 for a standalone with those mechanics, probably more than once so I can have it on PC and mobile. If you wanna make it more gimmicky (which, say what you want, can increase sales), lean in on procedural generation of the dungeons.
A gimmicky genre of games that's become popular recently are the tidy-up style games, where you have a set space (usually a shop of some kind) and a bunch of object that have been thrown around and need to be put back together. One of the more popular ones is actually about a library. If you wanna make bank off of a lot of your neuro-spicy player base, make one of those but with the actual elder scrolls books. You can just use the book assets ported over from Skyrim and that would give us a lot to work with, and if you add in modding support to add more books, including adding text, as well as the ability to expand the library in some way (through mods or through an in-game building system), you could easily charge a good amount for it. If you wanna be able to bump it up to at least $20, add more assets from Skyrim to be put away: weapons, armor, even junk items. Give them the whole of Whiterun's shopping district to organize, or perhaps otherwise hidden levels of the College of Winterhold. Or, for once, use the Bard's college for bard things, like musical instruments, sheet music, and musical theory texts. Though, honestly, it wouldn't surprise me if someone is already working on a mod for Skyrim or Oblivion Remastered with this concept in mind.
Games based on NPC jobs. One that I know would be heavily embraced by the fans would be a material gathering game, where you go out and get crafting materials, document them, prepare them if necessary, and sell them. So many really successful offline-only games have been built on this premise, with examples including Potioncraft, TCG Card Shop, Schedule I, and those mechanics are found in other hit games like Subnautica, No Man's Sky, and Stardew Valley. Blacksmith simulators have a decent popularity too.
Fallout:
Your minigames. Seriously, the hacking minigame, all of the holotape games, even the lockpicking minigame can be addicting. Throw those in a smaller format for cheap, especially if you can make it mobile, and people will gladly buy it as many times as we bought Skyrim, I promise.
Weapons maintenance and modification. You'd be surprised how many people would just love to take apart the weapons from Fallout 4 and/or Fallout 76, clean them virtually, replace out broken parts, switch out modifications, etc. If you want to make the gameplay loop a bit more meaningful, have them sell fixed weapons to be able to buy lots of broken ones. If you want to tie it into the recent increase in focus on New Vegas, just make them Gun Runners, maybe even let them repair, upgrade, and modify that robot that sells the guns in the game.
Starfield:
Variable Gravity Shooting Gallery. Seriously. Imagine the sheer chaos of trying to hit targets that float and bounce around after the first time you hit them.
Oh, and Poncle's survivor's-like collaborations are gonna make bank. If you have an IP that has lots of enemies and unique weapons that you can draw from, you might wanna see if you can collaborate with them. Every single survivors game and DLC they've put out has been incredibly fun and pretty popular from all I've seen.
So, hope this helps.
God Bless if you'll let Him, and have an otherwise great day.