So he is about to be Runetera's greatest seer ever...which according to my 2 min of research is someone who can see (shocking) the future. So what would that look like as a pre-void power if we made a champion that had "seer" powers?
Okay lets say that seer is a passive or passive ability. Maybe even a passive ult that he starts with based on what I'm thinking. Let's say you're mid farming away on Malzahar and playing opposite you is a Morgana. It's getting close to the time that the first ganks should be coming and all the sudden you see something in the topside river brush at mid. It is the enemy Lee Sin! But wait! You never put a ward in that brush, AND you notice the Lee Sin is either wearing a green skin that looks like the tracer that runs after Ekko, or that is your seer power kicking in. It tells you that Lee Sin is in the top river and headed towards that brush when he pierced the big 'seer' zone circle around you. Now you know its not yet the real Lee Sin and you are just tripping hard seer magic, which is great because now you can avoid possible destruction (after all you lack escapes right?). Since Malzahar is tripping hard, no one else can see this but him. Side note, if you think I am being to "street pharmacist" then read what happens to people when the void portals begin to form in parts of Runtera they literally hallucinate. Malzahar went crazy because he could see too much, so lets imagine what that's like for him pre-void. Back to Lee Sin! If Lee Sin continues into the brush within a certain radius of the premonition Malzahar originally saw, then it activates a 2-3 sec speed boost to Malzahar. This is because Malzahar was already ready to turn on the jets, he saw this guy coming and so whether Malzahar stays in lane or backs off, he gets the speed boost if the enemy fulfills the vision's estimate. But what if Lee sin was only headed towards that bush when he broke the seer barrier because he was punching the scuttle? Instead of going into the brush, where the seer powers foretold, Lee pulls the 180-turn still punching the scuttle but now following it towards the top lane. So instead of the green looking hologram of Lee Sin, the hologram updates the likelihood that Lee Sin will arrive in the bush before the vision fades out (the seer powers readjust to account for free will and all). Soon the hologram turns yellow, orange, and red before disappearing altogether. This tells the seer that actually Lee Sin is now moving away and will most likely not be in or around the bush.
Now our boy goes gets his ability to see what will happen in the future hijacked, and the channel he usually watches when he is doing the seer-thing is turned (by some voice in the void, might help understanding his new void powers if we know more about the owner of the voice that drove him mad), to a channel that basically plays a video of the Zerg overrunning Runetera on a loop. So now that he watches nothing but Fox News, all he can see are creatures of the void ripping apart the world around him, and he sees this so vividly that unlike Kassadin, it makes Malzahar say "If you can't beat 'em, join 'em." So again he sees void creatures all around that are not there but that could be or will be in some future. Also, hes's now on team void, so what is his role supposed to be on the team Zerg-like annihilation? Well, it could be that the driving force in the void needed more champions with which to ravage Runtera, however based on the current champions calling the void home, I don't think they need to turn champions in Runtera in order to take over. The void portals that open up around Runtera give little bursts of power, people hallucinate around them, and then "...if the Void is able to consume enough biomass to survive, it can grow and evolve into something ferocious and terrifying."
If we combine visions of an incoming void army, with the fact that there are portals which can bring something from the void into Runtera (if the portals are fed enough biomass), plus the idea that Malzahar was needed by the voice to do something that the voice cannot. Lets say that the being behind the voice that drove Malzahar mad can push on the places from the void side where the barrier between worlds is thin. This is what causes portals to start to form, but clearly he/she needs help from this realm to pull the door open while he/she pushes.
That's why they need Malzahar. Only his seer powers can predict where those portals that the voice of the void will try to open will appear in Runetera. Only Malz can see them, and only Malz can handle the void creature will pop out of each portal as he get's biomass to them. So basically Malz is Medivh.
To wrap up, Malz rework based on considering story line:
Passive - Seer's Eyes : Malzahar seer abilities allow him to see places where the barrier between worlds is thin. Portals can be opened on these spots once activated. Malzahar activates a spot by moving into the area around a spot. Once activated, when Malzahaer stands in the area around a potential portal location he gets +AP (or "+void damage") and any enemy (non-void?) that enters the location will be charmed (hallucinate) for X sec.
(Q) - Call of the Void : Once spots have been activated, Call of the Void allows only Malzahar to see a vision of the void creature a portal would summon, atop the activated spot.
Active - With enough biomass, Malzahar can create a portal. Portals summon different void spawn (This could be done in two ways. The void creature summoned could evolve or be a more powerful type as the game progresses or as Malzahar opens more portals, similar to old Cass poison evolution into more dangerous creatures. It could also be like 1 of 3-5 possible creatures randomly assigned to each new portal location from the start, but they scale, similar to the TF cards, but in this case Malz is stuck with whatever the portal location says will be summoned that time). After summoning a void creature Malz has control over it, it lasts for X seconds (In picking these creatures, I would suggest thinking of what an early evolutionary form of or lesser member on the family tree of the current champions from the void on the rift. Each could have an ability that reflects that relationship to Vel or Rek or Cho or Kog's group. Like for Vel it might summon a lazer shooting over the shoulder hovering drone-like mini-Vel that Malz can send somewhere to act as a hovering turret similar to Heimer's or Malz can keep/bring back mini-Vel on the shoulder as a second gunner similar to Lulu fairy. OR make a new void creature that learns/evolves as it is summoned and grows stronger. Maybe it could be some soldier or creation of the voice that spoke to Malzahar and drove him mad).
With the change to (R) so that Malz is not just an ult-bot, this has to be a major source of his damage to champions, towers and dragons/Barron.
(W) - Void Swarm : Summon voidlings. When voidlings attack a target they collect biomass. Malzahar stores biomass, which is used to open portals.
(E) - Malefic Visions : can stay but buff damage to champions (maybe make "void damage" a thing it has that is something different like how true-damage is different)
(R) - Premonition : Seer ability that grants visions of the future. Anytime an enemy fulfills the vision by passing through the vision [hit-box], Malzahar and all allies within X yards of Malzahar gain X movement speed increase that deteriorates over Y seconds. This effect can be triggered by more than one enemy, but never by the same enemy twice in Z seconds (basically like Nautilus's passive, he can root everyone, but there is a CD on each person as he roots them that prevents him from spamming).