That’s a very interesting question, Dan! As I am neither an expert on nutrition nor longevity, you should not place too much weight on my thoughts but … here we go!
First, as the overarching design principle, I would try to obtain as much of the nutrients as possible from actual plant foods rather than supplemented vitamins. This principle was introduced by the late Roy Walford and is followed by the CR Society. Their members tend to be very knowledgeable about nutrition and a search in their forum reveals the following rather unfavorable comment about the year 2016 standard Huel:
No soy (pea protein instead), and carbs and fiber from Oats, rather than carbs from maltodextrin. Otherwise pretty crappy ingredients + a multivitamin.
Second, I would look over the macronutrient composition. This is a vast literature and I have not followed it closely, but my impression is that the current standard Huel contains too much protein for optimal longevity.
Third, I would see whether the new formula could be fine tuned at the micronutrient level for longevity. This includes possibly including polyamines, which can be obtained (together with Vitamin K2 and the enzyme nattokinase) from the dirt cheap, though soy based, ingredient powdered natto. An alternative, non soy-based source for polyamines is wheat germ. I would also look at the ratio of branched chain amino acids to other amino acids.
To help readers understand the examples in my third - and probably least important - point, I end with providing two references that can hopefully help to elucidate the potential longevity implications of dietary polyamines and the ratio of branched chain amino acids to other amino acids. And yes, I know the evidence for the latter is currently weak but some ratio needs to be chosen in the final product.
Madeo, F., Eisenberg, T., Pietrocola, F., & Kroemer, G. (2018). Spermidine in health and disease. Science , 359.
Solon-Biet, S. M., Cogger, V. C., Pulpitel, T., Wahl, D., Clark, X., Bagley, E. E., … & Perks, R. (2019). Branched-chain amino acids impact health and lifespan indirectly via amino acid balance and appetite control. Nature Metabolism , 1: 532-545.