Well, he's similar to Barbapapa too, except he's the 18+ warforged version... Alex Mercer in Prototype is a result of some laboratory designed virus, MC is the result of some infection by a monster from another dimension...
They just happen to have a lot similarities, but there's no reason to assume they will follow the same growth pattern or will always get the same powers and abilities... They're both shapeshifters, that alone will result in many similarities and that concept is way older than Prototype... (wendigo, mimic, Zeus... there's many more shapeshifters)
Anyway, his abilities : inreased strength, shapeshifting and forming extra limbs or other bodyparts, memory reading, copy traits from animals and combining them and apply them to himself, forming metal parts on his body and limbs, fast regeneration, an inner voice that's somehow linked to his monster parent or tree... weakness to electricity, however that will most likely result in even faster regeneration in the future... a somewhat autonomically acting tail and increased spinal functionality... Can compress a body to tennis ball size, can change objects and/or their colors....
As for what to expect in terms of abilities gained from his next evolution? That's gonna be hard to predict, we know the potential for that is largely dependent on what powers and abilities his monster parent has (and we know very little) and it's also a matter of what his body and mind will activate... We know Jake gained his powers in a way that his personality affected how his powers manifested... We know Ella's powers are based in her eyes/sight, MC's in his spine, no doubt we will see different results even if both have the same parent...
So my prediction? MC's spine will evolve more autonomy, resulting in MC being able to handle more limbs just by instinct...
MC's weakness to electricity will turn into his biggest strength by being able to directly turn it into regeneration or even direct it into other functions in his favor. (shooting needles or forming "flesh" shields etc etc) And i expect his "copy and apply to himself" skills will grow more and more effectively... We may get some powers based on who we ate (or the eye from shop).
But some teleport skills and shields against mental powers or energy based attacks would be very nice too....
Ella on her fifth evolution gets some aura that at least partly does that, so i guess we'll have to evolve a bit more for that..
Either way, it will be better than the second evolution but we need the fourth and fifth to be even bigger....