An interesting AHC--Swap the popularity and adaptions of Legend of the Galactic Heroes with its writer Yoshiki Tanaka's other series, The Heroic Legend of Arslan. As a fan of both works (well, the anime adaptions) it's an interesting thought.
There's the obvious problem that the LOGH novels finished by 1989 while Heroic Legend of Arslan wasn't finished until 2017. I've only ever watched the two anime adaptions of Heroic Legend of Arslan so I don't know a good "stopping point" for an LOGH style OVA adaption, but maybe a lengthy adaption until the mid-90s and then a sequel (in place of the LOGH Gaiden OVAs) in the late 90s/early 00s. Maybe in TTL Tanaka finished Heroic Legend of Arslan earlier than OTL and it's one of those stories which never sees an adaption of its proper end.
I wonder if the Heroic Legend of Arslan OVAs of TTL would follow the weird psychedelic style of the OTL OVA (which is just straight up weird) or go for something more like Record of Lodoss War or even an aesthetic more like pre-FFVII Final Fantasy considering Yoshitaka Amano did a lot of work for the novels. And I could see that with an earlier ending, maybe there'd be a "remake manga" which would be both faithful to the source and yet "reimagine" it in certain ways, like the more recent LOGH manga (by Ryu Fujisaka) which has a faithful plot but plenty of weird character designs.
As for Legend of the Galactic Heroes, TTL I think we'd have the My Conquest is the Sea of Stars movie (the first LOGH media) changed into a 6 episode OVA (like how the OTL Heroic Legend of Arslan had 6 episodes) which would adapt up to the Battle of Amritsar (so the first 13 episodes). I think it would be cult hit in Japan and the West but would remain rather obscure. I imagine it would more or less be the same in style and presentation to the OTL OVA.
And I also imagine for this scenario that in the mid-2010s, we'd have a new LOGH manga, maybe more or less the OTL second LOGH manga, and like the second Heroic Legend of Arslan manga, this would get a similar adaption where the first show (25 episodes or so) would have the first arc being early Reinhard and Yang Wenli, like in the Gaiden OVAs parts of which were in the second LOGH manga and the second arc being equivalent to the first 13 episodes of the OTL OVA. Or really Die Neue Theses which this manga and adaption would be like, with all new character designs and VAs, and it would have a second season (like Heroic Legend of Arslan: Dust Storm Dance) dealing with the civil wars in the Empire and Free Planets Alliance to complete up to episode 26. I imagine that TTL this show would be far better received than OTL Die Neue Theses since there's nothing to compare it to.
And Heroic Legend of Arslan might have a remake in the late 2010s, maybe based on a manga like OTL. But unlike OTL, this version of Heroic Legend of Arslan (i.e. the TV version from the 2010s) would be poorly received by fans of the original OVA and only adapt the first parts of what the OVA already did.
Ishikawa Ken either lives long enough to finish Getter Robo Ahc, or else bequeathes his notes to his mentor Nagai Go, who finishes them in his stead. This then forms the basis for an Anime by Bee Media, becoming an hour long block with Mazinger Edition Z: The Impact!
Maybe it could replace New Getter Robo? I know that was a tribute to Ken Ishikawa since it included characters and settings from his other manga modified for Getter Robo while starting with a very accurate take on the original manga (albeit with the Oni instead of Dinosaur Empire and Musashi/Benkei merged as one character), but maybe instead we get a faithful take on the entirety of the manga? I could see a 26 episode TV adaption of the manga and I imagine if it was faithful and well-done it would be pretty well received in Japan and possibly the West (although sadly Gurren Lagann-tier success is probably ASB) although the ideal is separate OVAs for each arc of the story.
I think they'd probably change the cast from the original 70s TV version since most of the Getter Robo characters had their VAs changed for Getter Robo Armageddon and Shin Getter Robo
Between that and Shin Mazinger (plus other Mazinger-related stuff like Mazinkaiser and Mazinkaiser SKL) I wonder how much more 70s mecha could get remakes/reimaginings in that time period (in the vein of the Jeeg remake at the very least). Two properties in particular I imagine might be very successful due to the overseas market--a Grendizer remake (huge in Italy and the Middle East) and a Voltes V remake (huge in the Philippines). Although a series of successful Getter Robo OVAs is probably not the POD needed to save the OVA market sadly.
How will this change things like Super Robot Wars?
Getter Robo isn't near-entirely ignored in some SRW entries and in newer ones isn't almost always represented by Getter Robo Armageddon. Considering the timeframe, it means that Getter Robo TV (the 70s version seen in almost every SRW game) is probably never seen again (including the common take on Shin Getter Robo which showed up in the Alpha games for instance) and for the Super Robot Wars Z games we have them adapting the Getter Robo TV/OVA (whatever they go with), arc by arc. Basically like how SRW Alpha 2 had only the first half of GaoGaiGar (if it's a TV anime and not OVA).
Speaking of Getter Robo Armageddon, its first director Yasuhiro Imagawa (Giant Robo, G Gundam, Shin Mazinger) left after the first three episodes due to disagreements with the staff which resulted in a huge change in tone and style. Not entirely bad I think considering the show still ends up glorious, but I wonder what the effect on the Getter Robo franchise and the reputation of the show would be had he stayed, considering Imagawa's work has a huge cult following (although apparently his Shin Mazinger was disliked by some purists even though IMO it's a brilliant adaption).