While I would welcome changes to make it more competitive, it doesn’t really need to be changed at it’s core. People having problems with wasting the proc are just playing it wrong.
You can reliably only proc Hot Streak when you can cast an instant pyro immediately after proccing it anyways, so play like that and you’ll have a GCD’s length to react to the proc without wasting any time on it.
e.g. if one of your Fire Balls procs a Heating Up, use Fire Blast at the very end of your next Fire Ball cast and instantly shoot the Hot Streak Pyro. If Hyperthermia procs, you’ll shoot the pyro and have it’s GCD to realize it procced.
Your entire rotation can be adjusted like that, down to avoiding proccing Hot Streak with Phoenix’ Flames to avoid it’s GCD blocking you from shooting your first pyro as the proc comes up. Obviously exactly same story with Flame Strike.
If a GCD instant (like Phoenix) procs heating up, don’t instantly Hot Streak but wait for your GCD to be over before doing Fire Blast>Pyro (like almost instantly, just enough time for the HS to proc an instant pyro before firing it off)
During Combustion you use your spells so that you only use PF if you don’t have a HU or at least stand far enough away from the target that the travel time will eat PF’s GCD before HS procs.
If you play like, this the only chance to waste proc time is if it procs as the last mob dies. The same way you change your Fire Blast timings to keep Feel the Burn stacked during Combustion, you ahve to change the timings to maximize Hyperthermia.
If the talent NEEDS anything it’s a visual. Otherwise the gameplay is perfectly manageable and incredible fun. I played it through the entirety of Shadowlands to great results.
Maybe add a bonus damage modifier on those casts to make the talent competitive, increase it’s proc chance … yknow… tuning. But strictly gameplay wise there is entirely no problem whatsoever with it’s usability.
It’s wild that so many people think you need robot like reflexes to maximize it when you literally have full control over when it can proc and the condition under which it can proc wants you to press the same button as the proc itself (Pyro/Flamestrike). Yea you don’t know when it procs, but you can totally set the times it can proc up so that you will press the correct button to react to it regardless of whether it procs or not. There is plenty of specs that benefit from you being able to react within a GCD’s notice (to continue dishing out Pyros instead of going back to building one up) and a - say - fully hasted .75s GCD is not inhumanely robotic reaction times. (On average, human reaction time takes between 150 and 300 milliseconds. For your brain to register anything anyways, acting as a reply to such a stimuli takes about twice as long with e.g. slower car drivers needing .8 seconds to hit the breaks when they need to, which isn’t far from the .75 GCD cap. If your reaction time act on stimuli is less than 1 second you may be slow and should maybe not drive a car according to howacarworks dot com)
Personally - again as someone who played that Legendary throughout all of Shadowlands and loved the ever living hell out of it - I would be super sad if it got fundamentally reworked instead of getting buffed up to par.
I can see you have not really played with it because this would be a horrible horrible nerf both fun and DPS wise.
With double Bloodlust or PI or just normal bloodlust, a haste trinket proc/use or anything like that you can dish out SO MANY PYROS in a single proc, you’ll very often reproc it from all the Hot Streaks you generate while absolutely chaingunning some poor mob into oblivion. It’s SO MUCH FUN. (sadly no additional procs for flamestrike since no HS )
Your suggestion would kill the talent.