I personally believe a sliver couldn’t hold a spark to planeswalk…then again…that would be awesome.
And as for being smart enough, hold your tongue! Their hivemind is simple, yes, but they are a very intelligent species, surviving when no one else could. They are sentient among their own ranks, and know how the world works. They are intelligent beyond reason. Yes, they aren’t advanced, but their Legends are. The Queen, Hivelord, Legion, and Overlord are all near sentient, if not fully sentient and can control entire armies with a thought. All the Legends have great promise and skill.
What about a sliver walker who just makes different sliver tokens? Like his +1 could be like get a 1/1 with all slivers get +0/+1