I cannot fathom a way in which a level cap increase is anything but a minor disaster minimum and a major disaster in all likelihood.
This is not an MMO. It already has a constant, larger demand for leveling than MMOs via the arrival of new characters in player accounts necessitating leveling each from scratch each time. Compare that to a one time burst of leveling for one character with optionally- but not fundamentally required in the same way genshins content demands at least 8 characters- of alts later. And even for alt a holics like myself the number of classes in a game only goes so high while a gacha can keep adding indefinitely. Furthermore, expansion leveling is typically linear/consistent in time expected with prior expansions. Genshin's leveling jumps grow in all required materials for each batch of 10. Taking into account both the growing nature of a player account and those increasingly more severe requirements, conflating the two genres would be comparing a linear exercise with a brutally exponential one.
It's also worth noting that mmos usually take great strides to make things easier for both alts and players clearing non current leveling or come under major fire from players for not doing so and then end up doing so anyways. It's certainly possible that a Genshin level cap increase could come with reduced requirements for all prior tiers such that the total leveling cost is the same as now but then... that begs the question of why do it in the first place?
Maybe your answer to that is the potential xp stockpile. Old players wouldn't reap the benefits of reduced leveling requirements for 1 - 90 for their existing characters but would have to contend with leveling to 100 for potentially many characters, disproportionately targeting them as the likely holders of those stockpiles perhaps. For folks like myself who've been battle pass users and played for significant periods of time- played since day 1 myself but took two extended breaks- we may have a stash of xp mats yes. But that's not for lack of leveling characters- its because you get so bottlenecked by so many things after and during the xp mat expenditures you're never going to get through them all faster than youre going to.... feel like running around the sumeru desert a billion times to catch a hundred plus scarabs AND farming a resin locked boss for a ton of mats AND farming a potentially different resin locked boss for elemental gemstones AND farming at bare minimum an acceptable artifact set AND farming weapon ascension mats AND farming talent leveling mats each time. Even with a bunch of new characters coming in, accounts just don't get to use the xp mats fast enough when gated by everything else if you're using a battle pass and I don't think adding a new tier will change that since it will also come with a new set of those other bottlenecks too. At best you'd see a brief one time dip in those stocks only for them to build back up again so long as the ratio of xp mats vs flowers/monster mats/boss mats/elemental gemstones/everything mentioned above holds the same.
To piggyback off that point, since I've come back I've gained over 300 wishes and not wished once. While that is partially due to a desire to see what Natlan has in store, there have been appealing characters for me to pull on but every time I've felt tempted, I've looked at the incredible backlog of stuff to build I still have and not been able to rationalize pulling a character that I would not get to building- without exaggeration- for months. And my account pre hiatus was a veteran one where I used my resin constantly building someone so it's not for lack of effort before. They're literally damaging their incoming revenue from me as is with the incredible burden that is building any one character. Stacking even more requirements on top of this will only exacerbate the effect/further reduce my desire to pull and any temptation to spend if i even stay with the game.
Then we consider the endgame. Players will either be gated from participating in the highest levels of abyss in the initial months following a level cap increase or they'll have to slowly increment the levels of the abyss enemies up over a few months, creating a arbitrary tuning challenge for themselves for little gain. If they do at least do the ramp and don't just effectively end participation in the mode right away for a time, players willing to buy resin will stomp it. But I guess kind of a moot point since people throwing extra cash at the game can stomp it anyways in any number of other directions.
Finally, the only actual potential silver lining I can see to this is maybe some sort of positive balance change? But then I actually think that's less likely than the opposite happening. Base stat increases will favor five stars over four stars and while, yes, that may nerf some of the more tired 4 star choices, I don't think that is a good thing for the game in general. It will also favor reaction based/hp scaling comps- some of the more favored comps in the game as it stands currently. New passives could allow them to tweak units one way or another.... but if theyre retuning the entire games roster across the board both in stats and in passives, it seems unbelievable that they'll manage to create a better balanced game with that many moving pieces at once than the fairly well balanced game Genshin currently is for a gacha?
And if they're willing to do a balance change, they don't have to staple it to level cap increase. Once the can of worms is opened, you could just slide it into character kits as is. I don't think "hiding" it in a level cap increase fundamentally alleviates any of the gacha ickiness at all. It just adds another thing you have to tune right. So even if I came away liking the balance changes, it still could have just been added without the levels.
Speaking of that legal liability/change to sold products this would represent- even if they don't do something as drastic as adding new passives and "merely" extend numbers upwards, the fact is that either you tune content to account for the new numbers and every characters effective strength shifts due to the aforementioned scaling concerns or you don't and the game becomes a boring stomp. I think even in that scenario where its just stacking numbers on unchanged kits they should be open to complaints legal or otherwise for having sold characters that now no longer function as understood. While I don't know if I'd have any real ground to stand on, I'd at least look into seeing if I could receive any sort of compensation and likely just leave the game altogether, and right before the region I most anticipated/came back for. :/
This is certainly early to be fair. It's just a leak and not a confirmed one. We have no details. Wait and see is always a level headed and fair approach but just going over everything that would have to considered, changed, and executed correctly, I cannot envision this being good for the game. I didn't want to bail on it so soon but short of them impressing me with it in ways I can't conceive, I'll probably be quitting. :(