Well, as we can see in the latest chapter, Ichigo has a very hard stand against Yhwach who assumes to see all the possible futures and transform them through his will. By e.g the transformation of the Soul Kings palace, he has proven that he is not joking about his powers. But there has to be something like a blind spot or otherwise no one could stand a chance against him.
I could imagine that Haschwalth might be part of this blind spot. As I said above, Yhwach assumes to see every possible future and can change it to his will but how can a being with self-proclaimed "absolute sight" be really sure that he is not missing something ? Just because you can't see or sense something doesn't mean it doesn't exist or could exist. Like a sand grain which is hidden by the countless other grains Yhwach is watching if you want to use the metaphor of the Quincy king. And even if he is not missing anything might he be arrogant enough to assume that there is a possible outcome which is so unlikely that it doesn't need his full attention ?
Back to Haschwalth: What if this outcome is the defeat of the Grandmaster of the Sternenritter which Yhwach did not foresee or let slip through his attention ? Yhwach called Haschwalth his "other half" and well, if you lose your other half - that hurts. And his death might even have more impact on Yhwach: Normally, "The Allmighty" changes his possesor at night. But if Haschwalth would die before the next night ... where would "The Allmighty" go ? Would it vanish ?
Wow... this is actually a pretty sick theory. But there is something I must point out. If you recall, back in chapter 633, that Yhwach is capable of retaining "The Almighty" without Jugram's help. However, at that time the Sternritter did not exist, and my theory is that Yhwach has trouble maintaining full control over "The Almighty" during his sleep. That is why he needed Jugram; a vessel powerful enough to hold that overwhelming power, but also to be capable of giving it back when Yhwach can control it. Note that it is only in his sleep that Yhwach is capable of passively absorbing the loose pieces of his soul. For all we know, he does not require sleep to maintain his vitality (after all, he is a son of god), and only uses it to power himself up by retaining the loose pieces of his soul. If he maintained "The Almighty" during the night, he could have unwillingly decimated the Sternritter, leaving him without the bulk of his invasion army. But now, there are only two Sternritter which we can be sure are alive: Gerard (note that he cannot lose power to Yhwach in that way, as his power never belonged to Yhwach to begin with) and Jugram (and if my theory is correct, Yhwach has no further us of him). The rest of Sternritter are most likely dead, and Lille himself said he "lost his divine powers", leading me to conclusion that he no longer has anything Yhwach can make use of.
I think, the story back in chapter 633 takes places some time before the plot when Yhwachs powers were not fully unsealed as it's said in the Kaisergesang. Therefore the problem of controlling "The Allmighty" might have been not so big - if there is any problem controlling it. But the theory that Yhwach needs a vessel to maintain the control of the power at night sounds pretty decent for me.
Nonetheless, I still think that there might be some more about Jugram than "just" being a vessel: Yhwach called him his "other half" ... maybe Haschwalths death would have more impact on him (but who knows :D).
Maybe this could be an opportunity when Yhwach realises his loss (I don't mean that he would be sad or something) and is lacking in concentration for a few precious moments. According to the latest chapter, Uryus Schrift could be the key to it aswell. But they all need some rest or back up or the fight is pretty much over.
I could be wrong, but I think Yhwach called Jugo his "other half" not to be literal, but because they're unique in that they can give, not take. So the 5 War Potentials together may be the key to taking him down
After 900 years the Quincy kings heart starts to beat again, 90 years later he regains his interlect and finally after nine more years, he regains his full power - the day when Yhwach regains his true power is the day when Ichigos and Uryus mothers died and chapter 633 was definetly before that time so Yhwachs true power was sealed at this moment. That doesn't mean that he couldn't give parts of his soul to other beings and that he wasn't dangerous as hell but his true ability - The Allmighty - was out of his reach. So (theoretically) he might have searched for a person like Jugram who can give parts of his soul to others too in order to use him as a vessel.