First step to rebalancing Ryze: Revert Buffs
Patch 6. 17 Range increased to 1750 / 3000 from 1500 / 3000.
Realm warp is a strong tool in pro play, but underwhelming in solo queue. Many solo queue players skip leveling ultimate at 6, investing points into it around levels 11 and 12.
Additional hypothetical change: Increase channel time from 2 seconds to 3 seconds. Gives more time for enemies to react in high levels of play (baron rush, flanks), more time for allies to take notice in lower levels (is it suicidal ult).
Edit: For clarity, I KNOW Ryze's win rate. I kept up throughout the years and know it hasn't reached near 50% since season 5. His ultimate is very good in premades/LCS, but doesn't bring much solo carry potential. Remove that buff, put that buff elsewhere.
Unless you think that this Ryze ultimate buff outweights the nerfs to his movement speed bonus, shield, root duration, spell flux cooldown, and fluxed overload damage.