In that case:
tank: Wits, Mallet, omen, Warmog, GA makes her pretty hard to kill, but not a lot of dmg.
Gauntlet and Frozen heart have been mentioned too.
For magic res: spirit (CDR), banshee or if you want some more dmg merc scimitar.
For an offensive build... Hydra, an ats item like wits, zephyr or malady maybe static shiv. + more dmg.
BT, botrk + bc combi...
If you want to try something you could go for hybrid items, Trinity, hextech, rageblade...
Its not that she has a small pool to pick from, its that she doesnt make really good use of them in certain cases.