I'm not a speaker guy so i can't talk about that.
The Diana versions are not flagships, the 1266 is. And with the 1266 iterations, the community was angry, because there were so many iterations of them in a row with steep price increases. Even later when the TC was the newest 1266, it was 5k and got a price increase over time just by existing.
The Arya versions did not increase in price, rather the opposite: The old Aryas were price dropped and you could still buy them for some time for a very attractive price, the newest version just replaced the previous one. I also think there is not a single "major jump" in Hifiman headphones if you only look at one model and then the model that is priced slighly higher. It's a gradually slope. Maybe from the HEKSE to the Susvara if you have the amplification for it.
I have to say though, i'd rather have a company try to improve step by step than completely change their lineup (and house sound) like what Audeze did.
Why would we expect a new Susvara (or Utopia, 1266, etc) to be any different?
Because it's been nine years since the Susvara release and they are de facto the pioneers in the planar headphone world that always have the best technicalities for each price point. I agree that sound is subjective, but to me Hifiman is the cutting edge company when it comes to planars (which i like more than any other driver variant). And then they went for closed back headphones and it went to sh!t. Also, the price seems higher than Susvara, that should be an indicator.
From what company could we expect the next big thing? Raal maybe, but the Immanis is not a planar. Audeze changed a lot and when Sony bought them they made one of the best headphones of the decade in my opinion, the Maxwell, but it's for a completely different use case. Fun fact, the headphone that gets the most time on my head is the Maxwell, i can listen to music while doing chores etc, and have a sound quality that's only slightly below a Sundara.
Sennheiser doesn't do planars and doesn't seem to make any progress with their (in my opinion) laughable revisions of the HD650 (and even the other new one, HD490). People want a successor of the HD800S for years, and a cheaper, not so fancy HE-1 for 8-15k, but nothing happens.
While smaller developers pop up and they release excellent planars (Erzetich, Modhouse), they are not the best of the best. Abyss did that, which was a miracle.
I personally think that we reached a plateau for planars, because nothing came out that touches the Susvara (or 1266) for years. If Hifimans new flagship does not improve on that, i truly believe nothing ever will. The only step up would be to switch to electrostats or more obscure stuff like ribbons. But because i really like the planar sound, that might not be an option for me.
So yeah, if Hifiman can't do it, probably noone else can.