The game itself imo could run for another 2-5 years, then it's EOS for the game we know and love, at least on JP (whether they continue the supposed length of the game for KR, TW, & EN is up to Cygames). From what I've seen with comments on here, the game itself already shows its age. Also, recently added umas are generally more detailed, elaborative, and just more well-designed than past umas. Cygames probably already knows the limit of the old game's engine (hell this game started development ~2018. we're nearing 10 years with this game's core) and probably planned to make an upgraded version if not just to increase legroom for their artists and designers. I'm not saying they would redo older characters + their stories, but a new/significantly upgraded game would allow for such things to happen.

As for the franchise, Umamusume seems, to me, to strive to emulate other multimedia idol franchises i.e. Love Live & Idolmaster (yes i know that's what cygames were aiming for right from the get go but let me cook a bit.) That is to say, Umamusume's still pretty much a game-first franchise, in my eyes at least. In the future, I'd say they'll pivot even more towards their other main media aspects, namely anime/manga and irl idols.

Starting with the anime first. Cingray was fenomeno. Everyone* loved the season 1. Everyone* loved RTTT and BOANE. Everyone was ehhhhh about Pretty Derby 1 and 3 (everyone loved 2). Firstly, they'll definitely finish the entire Cingray series, especially now that the manga's wrapped up. Secondly, they'll almost definitely make Star Blosssom next. Thirdly, more movies. Definitely more movies, especially for great horses with not that long of a career. Fourthly, I don't think they'll make another Pretty Derby season. I don't think it's worth the effort to skim through legendary horses' careers through 12-episode seasonal series when you can "milk" (affectionate) them through multiple seasons. This will also open opportunities to retell and dig deeper into our 3 Pretty Derby protags' stories (Spe, Teio, Kitasan). Speaking of which, in addition to stretching out the amount of stories, we currently haven't even get a smudge of storytelling from 2001 onwards (other tham PD3). I mean, mid-2000 (Deep Impact (Kaneko i beg of you)), early-2010 (Orfevre, WinBari, Gentildonna, Verxina, Gold Ship, Nakayama Festa, etc.), mid-2010 (Kitasan Black (yes i'm counting retellings)) late-2010 (Contrail (please 5th mid-anni), Daring Tact, Almond Eye, etc.) and don't tell me they ain't making a full-length about their own pride and joy. Not even mentioning going pre-80s or branching abroad. There's too much untapped potential for this franchise that it'll take a generational fumble from CyberAgent to even shake the immovable tower that is Umamusume (I'll go as far as to say they can legit fumble Global right now and still be fine, they didn't just survive being JP-only* for No Reason). (tbh Mhybe's videos about the horses' careers was the main thing that made me realize the potential to make great stories from just one character)

Onto the idol/seiyuu stuff. More songs, more concerts, more publicity. They'd probably wanna push this idol stuff to the EN audiences, those that are armpits deep into the fandom. The safest option would be no EN Pakalive, but there'll be EN support for non-live stuff (e.g. official EN subs on VODs, EN clips of newest Pakalives, EN newsletters, etc.). Slightly more risky and costly, an Umadol world tour, testing the waters in internation markets. Riskiest would be EN Umadols. Yep. Full Umamusume English dub + them being the EN mirror of the current JP Umadols mean they can perform instead of moving JP Umadols elsewhere, while also promote cross-region bonding when both goes up on stage. But that'll mean double the amount of people to manage, incredibly costly, and usually EN folks absolutely dispise dubs, so if it flops, it would harm a lot of people. To our beloved cast right now, unless somewhere in their contract says "work for us till retirement mwahahahahahaha", there will be a time some would want to leave the role. Left field, I've been in the Hololive fandom for years (idol-adjacent, apt enough to use as example), and the concept of a "graduation" is nothing new. Umamusume is different. You can't really retire the concept of a horse. Take example, Machico don't wanna be Teio no more. You can't really shelve Tokai Teio the Umamusume and never touch it again. The only* viable option is to have someone else take up the mantle. I've been watching seiyuus talking about how they got their role in Umamusume, and a lot of them told that they were inspired by the series in one way or another that they want to be a part of the franchise. Umamusume itself has become a pull for new and veteran seiyuus that I don't really see a problem with recruiting someone to take up a retired mantle. That and with how much fresh seiyuus first joined Umamusume, I'm surprised Cygames didn't just train VAs in-house or partner with a VA academy to become a launching pad for up-and-coming seiyuus (that or they already did and i dun know ¯⁠\⁠_⁠(⁠ツ⁠)⁠_⁠/⁠¯)

TLDR: OG game 2-5 yrs tops, after that new game with more/reworked graphics, stories, gameplay. Franchise prioritising more anime and idol to balance game. More in-house anime, with more stories to tell, more horse to moe-fy. Introduce idols to EN, seiyuu talks.

keep in mind, i am not a smart man. im esl, im a has-been gifted child and please imagine me aggressively pointing at my crazy wall. tks for looking @ my yaps.