RIOT's Xypherous reveals the changes to be done to Tiamat in order to make it a viable choice for melee champions.

The new Tiamat will keep the bonus damage and health regeneration from the old version but sees its MP5 removed. Furthermore, the passive bonus of the old Tiamat is completely reworked and now the Cleave deals a % damage based on your attack damage and not damage dealt (eliminating the need to stack crit chance to maximize Tiama efficiency). An active skill with 10 second cooldown has also been added that'll deadl 100% of attack damage to enemies surrounding the champions.

All in all, the revamped Tiamat looks like this:

Tiamat
(Melee Only)
+50 Attack Damage
+15 Health Regen per 5 seconds
UNIQUE Passive – Cleave: Your attacks deal up to 60% of your Attack Damage to units around your target – decaying down to 20% near the edge.
UNIQUE Active – Crescent: Deals up to 100% of your Attack Damage to units around you – decaying down to 60% near the edge – 10 second cooldown.

The item will also be upgradable into Ravenous Hydra, which is, simply put, a life stealing version of Tiamat.

Ravenous Hydra
(Melee Only)
+75 Attack Damage
+15 Health Regen per 5 seconds
+10% Life Steal
Passive: Damage dealt by this item works with Life Steal.
UNIQUE Passive – Cleave: Your attacks deal up to 60% of your Attack Damage to units around your target – decaying down to 20% near the edge.
UNIQUE Active – Crescent: Deals up to 100% of your Attack Damage to units around you – decaying down to 60% near the edge – 10 second cooldown

Xypherous also hinted that AoE damage items for ranged characters will be coming out soon.

Source: EUW.LeagueofLegends