I started league in season 4 and picked up kassadin in season 5. I thought he was a good champ not to broke and not unplayable. I could live with the changes during 5.3 but 5.4 is just to far (unlike his riftcrawl). I have some ideas, but I wish I had my old friend back.
If there gonna keep the rift crawl as it is that could be fine. What they should do is give him back his silence and make his E 4 charges instead of 6. This way he still has to flank but trying to get his opponent will be a lot easier. Now he will not be Able to go in for clean ups and get out but do an Assassians job on taking out the weak.
