I probably spent way too much time on this, but since this seems to have been a notable suggestion for awhile now with not enough elaboration, I thought I'd give it a go. If this gets negative feedback, then I think its safe to say that we can wave goodbye to the chances of ever seeing an Zamorakian Hasta.
The concept I think is pretty neat. I've never played OSRS so I have no experience with how powerful the Hasta is. From reading the stats and that there are no penalties for converting it, however, I think it's safe to assume that the Hasta is quite powerful.
The one problem that seems to reoccur with the suggestions towards a Zamorakian Hasta is purely allowing us to convert the weapon to a one-handed form with no penalties to opening up the Shield Slot. This is how it works in OSRS, sure, but to elaborate on this:
Elemental Whip:
Attack Bonus: 100.
Strength Bonus: 91.
The Elemental Whips are currently what most people consider to be the strongest one-handed weapon, give or take. Not referring to special attacks, just pure damage output.
Zamorakian Hasta (With no Penalties):
Attack Bonus: 150.
Strength Bonus: 149.
Seems a bit overpowered for a one-handed weapon, wouldn't you say? Toss on the Arcane Spirit Shield's strong Attack and Strength Bonuses and you have yourself an incredibly powerful weapon with access to Defense Bonuses.
There's no negatives to it. Just slapping on a shield with those stats makes it quite OP. Considering the bonuses that you do get from these shields, this would make a Zamorakian Hasta outclass basically every two-handed weapon on stats alone. No good. These suggestions lack stats, which for something like this plays a big role IMO.
Ideally, if a Zamorakian Hasta were to exist, it needs to be balanced accordingly. It should be something that out-classes the Dragon Hasta while being comparable to the other Tier-80 Weapons (For this argument, Elemental Whips), but still be out-classed by the Zamorakian Spear.
To get my "Ideal Stats," I'm going to take the average of the Dragon Hasta and the Zamorakian Spear and compare them to that of an Elemental Whip. I'm going to mainly focus on Attack Bonus and Strength Bonus, but I'm also going to mention Attack Speed:
Attack Bonus: (150 + 87)/2 = 118.5, rounding down to 118.
Strength Bonus: (149 + 79)/2 = 114.
Attack Speed: Something in between the two would be the most viable choice. This sort of goes against the norm of hastas, but for the sake of making it balanced would make more sense IMO.
Only by the average, these stats are still a bit high, so I'll toss in some lowered stats:
Attack:
118 x 0.95 = 112.1, rounding down to 112.
118 x 0.9 = 106.2, rounding down to 106.
Giving the Hasta a range of 106-112 makes it noteworthy, comparable to the accuracy bonus of the Elemental Whip at 100. With Arcane, this gives the Hasta roughly 121-127 Attack Bonus, compared to 115.
Strength:
114 x 0.95 = 108.3, rounding down to 108.
114 x 0.9 = 102.6, rounding down to 102.
114 x 0.85 = 96.9, rounding down to 96.
The original range of 102-108, however, feels a bit too powerful. 96-102 Strength Bonus seems more comparable to the Elemental Whip's 91. With Arcane, this gives the Hasta roughly 106-112 Strength Bonus, compared to 101.
This would give us a weapon that is statistically superior to the Elemental Whip in Attack and Strength, but by averaging the Attack Speeds, notably slower. Maximum Damage Output vs. DPS. But it still floods Melee with more weapons that we might not need, which is something that could draw concern. I'll also bring up Korasi, which tends to be forgotten about in the tier-80 category, which might get outshined by this in almost every way.
As far as obtaining a Zamorakian Hasta, it should be through some sort of Conversion Kit like usual. Degradable can be debated, but making it untradable in this state, dropped as the Conversion Kit upon death, would be ideal. Potentially sinks Zamorakian Spears, win-win.
My original thoughts on obtaining the Conversion Kit were through K'ril Tsutsaroth, but I don't think we need to flood his Droptable with more drops. Perhaps keeping it with who you get the Zamorakian Spears in the first place would make sense.
Mithril Dragons have two Very Rare drops: The Zamorakian Spear at 1/200, and the Visage at 1/1000. Perhaps sticking the Conversion Kit in there at 1/500 would work? Just some food for thought.
That's all I got on it though. I tried to elaborate as much as I could, but I'd love to hear what you think.