As for items that would only help burst casters, the only item right now I can think of is an AP item that has an active ability that does:
-For Xs (let's say 3s or 4s, cooldown is from 60s to 120s), your AP is amped by 25-30% or a portion of your spell's damage is dealt as true damage (lets say 30%)
-Every Xs (lets say 15s-30s), you get a counter, up to four counters (for QWER). You can then hit the active and for 5-10s your spells will have their damage amped by 25-30% or do the true damage thing again, each spell cast consuming one counter.
Having AoE and DoT penalties can be put in as well if so desired.