Not necessarily saying reboot mind you. But maybe a new mechanic or a complete overhaul.  It can be a reboot if you think a series needs one but in what way?

I think Gears 6 will need to do something a little different and no a bad open world is not it. I don't think it needs to change its entire identity but I think it's time with modern hardware to update enemies a little. What I mean is have you ever played Binary Domain? If not shooting enemy limbs like shooting a leg they hobble or crawl. Shoot an arm and they can't hold a weapon. Shoot the head and they go nuts before exploding. Now those are robots but with the Swarm some of that can work. Limb damage. Affecting their movement and the weapons they use. Better AI as wack a mole needs to evolve. Keep the game linear and add a lot more enemy variety. 4/5 lack that. Lastly, crank up the gore to Space Marine 2 levels.  Yes, I know the prequel is due out this year and I look forward to it but can only go back to Marcus well so many times. So I am thinking about the future. Kait or whoever is in G6.

Super Smash Bros. needs at least a soft reboot. The roster is not sustainable. Some characters need to be cut, added, and some veterans need to be retooled.
Paper Mario needs to go back to the format of the first two games.

Splatoon needs a refresh; 2 and 3 were excellent games, but they were highly iterative and didn't do much new or surprising.

There's nothing strictly wrong with that as they're still a lot of fun, but for the next entry I feel like they need to change things up to avoid the series stagnating. Use the added power of Switch 2 to have like 4 team matches or something like that.

I'm onboard with your Gears 6 refresh ideas. I would like to also add more variety to a body being maimed in executions like when you bunch someone 10 times on the ground, there should be bruising and blood on their face. Sometimes a body crumbles into pieces after a curb stomp or whatever which doesn't make sense. I would also like to see more off the wall weapon variety like the digger and buzz saw.

Halo needs a refresh too. While they did some cool stuff with Infinite, it could have been much better with their open world design. Go back to a linear story with more big fights like we got with the scarabs in Halo 3. Have bosses like in Infinite. Time to bring back the flood with some truly horrifying moments. More vehicle and enemy types too. Also, fix the damn explosions. I hate the way phantoms explode in Halo Infinite. They just evaporate. Halo 3 and Reach had great explosions. Overall, gameplay is great in Infinite, but add more weapon variety and equipment.

I mean, how long do we need to consider a game series into "the long running" ones ?

Imo, I'll be glad to see the Xenoblade series changes some aspects when they'll take their next gen game. Takahashi is likely to devoid as much referentials to his past work (Xenogears,Xenosaga,etc ...) for a new game to start a new saga.

Anyway, sounds plausible as of now that they'll stick to a possible sequel to the upcoming Xenoblade X we'll see ...

Hum otherwise, I guess StarFox is any as good to give a shoutout for. It prolly can't stay true to form as a simple arcade shoot'em up. Maybe something like Starlink-ish/Adventures hybrid wise could kinda work ?

CASTLEVANIA.
Do I need to say more... Okay I will anyway - Imagine a 3D metroidvania (like the Metroid Prime of the Castlevania series) with open-ish world full of secrets and imagine the action (lords of shadow, souls-like eldernring) style of gameplay (but not too difficult unless it's a secret area or a special boss).
I wouldn't mind a Lords of Shadow 3 or a new traditional 2D mentroidvania but in a 2D-HD style like octopath traveller/ DQ3 HD-2D Remake style of graphics.

As for any other series'. Any great 1st-person game series' moving towards VR would be great too (Bioshock, resistance trilogy remaster, mass effect trilogy remaster etc).

I think Bomberman would be good to see a new pixel style game but in a 2D-HD (Octopath Traveller style).

Oh, (I might get some criticism for this one)... ZELDA.
BoTW and ToTK are really good in all, but I think it's time to change the formula (even if they go back to their traditional 3D Zeltroidvania-ish roots, but maybe with a twist to keep it fresh). They don't necessarily have to go back to their traditional-3D roots and I'm sure they will do something new next Zelda anyway but I don't know what.

Also I wouldn't mind seeing a Pokémon game where you can play the Pokémon with action combat in an open-world. You play the trainer and throw the pokéball. Then play the Pokémon when it comes out of the ball. You can switch between the trainer and Pokémon at any time and the Pokémon will auto-attack, command-attack, follow etc.

Also after GTA 6 gets released. I hope GTA 7 takes full advantage of A.I for its NPCs. Imagine talking to an NPC through the microphone of the game controller as if it's ChatGTP. Asking them where a location is or telling them a fake story about another NPC who called them a rude name and seeing them go up to them and start a fight. Every NPC will have literally a mind of their own. You can ask them about where they work, family life, fav colour/music etc.
True A.I in NPCs will be a thing of the future, it'll make games feel fresh again, it could even create new entire genres. Games would be unpredictable again, even to the developers witch could lead to some fun secret glitches that would be shared online.

All I want out of the next Castlevania is a 2D linear game like Castlevania IV or the NES games. Not saying looks like those. But classicvania. Already 45 million Metroidvania games out now 2D and 3D. Half of them are Castlevania itself. Besides Bloodstained is the Metroidvania SOTN series now. No. Either 2.5D or stylized 2D visuals. Maybe Lizardcube or something hand-drawn. Just no more SoTN clones. I have waited 30 fucking years for Castlevania to return to its old form. CVIV is still the best in the series. Linear. Dark. Atmospheric and gothic. LoS was the closest we came to replicating that.

Smash will get less characters in the next iteration of the series. They already said that it probably wouldn't even be possible to bring back all those third-party characters, so it will shrink du to this already anyway. And hopefully they'll remove some Fire Emblem characters and some Link variant (young Link or toon Link) and retool Zelda with the new abilities in her own title.

Of course, they will also bring some new characters to the mix. Personally, I'm hoping for Chibi Robo to make the cut next time

I'd like to see them try a 3-D version of Smash Bros, using Capcom's PowerStone as a template.

Smash was amazing with its wide varity of charakters. Give me Smash in 4k/60FPS- likley it will be anyway (3DS- 240p, even the WiiU runs Smash at 1080p/60FPS, also the Switch), its highly unlikley Smash won't max out the Switch 2 as it is able to do 4k in docked mode. Personally i only game in Handheld mode so 1080p/60FPS is fine for me and thats what it will be.
Interesting will be the question if Switch 2 is compatible with Amiibo. But i have a sort of feeling they gave up on them. Last year had what? 7 amiibo or so released?