Ignite + Barrier in an easy lane
Ignite + Flash in a hard lane

Buy enchantment boots whenever needed
Sell Sightstone once inventory is full

SITUATIONAL
Rylai's and Warmog's - When and ad and ap or hybrid is fed on enemy team

Randuin's and Thornmail - When an ad is fed

Banshee's and Visage - When there is too much cc or an ap is fed

Sword of Occult - if snowballing and enemy team are stacking magic resist

Mejai's - if snowballing

Mobility - If bot or top lane is losing/feeding

Swiftness - If all lanes are pushed too far and jungle is only option

Mercury - When an ap is fed

Tabi - When an ad is fed

Have the two items replace GA and Void

Spellvamp is a must on kat, her q bounces on each minion which heals about 10-20hp early game which will let u stay in lane for a very long time.

All the ability Power is great so spellvamp heals more.

If u dont agree with these runes another good one is to replace all the flat ap runes to all scaling ap runes which is just as good.

^ for against ap mid
change the 3 in mr to armor if against ad mid

if u think u will be in an easy lane go 15/0/15