What if Viktor's skills evolved based on usage, using certain criteria per skill?
Basically scrap the Hex Core entirely and instead give each ability a little bar above it, like Kai'Sa's abilities have.
These bars fill up based on usage rather than bonus stats, though, since Viktor's thing is research and development, adaptation and improvement. Seeing how his skills work, and how he can make them better.
- Siphon Power gains a little bit on cast, plus extra if the shield mitigates damage, and maybe a bonus if the empowered AA hits an enemy champion.
- Gravity Field gains points for every second that at least one enemy is snared by it, and bonus points if a champion becomes stunned.
- Death Ray simply gains points based on the number of units it hits.
Chaos Storm evolves when the first three have evolved, as usual, and Viktor's staff still changes color with each evolution unlocked.
Makes tank Viktor's game plan a bit more involved than just "back at 1150 gold and become obnoxious".
I don't have any numbers for you, just a concept.