Iunno I have an idea
+55 AD
UNIQUE Passive: +30 Armor Penetration
UNIQUE Passive: +15% Spell Vamp
UNIQUE Passive: +25% Cooldown Reduction
UNIQUE Passive: On cast, your next basic attack deals bonus damage equal to 100% of your bonus attack damage. (10 second duration)
Key point here is Spell Vamp to encourage more spellcasting, so now your Pantheon can spam spears to heal!. I added the last passive because some casters get autoattack modifiers, such as Renekton (W), Talon (Q), and Riven (Passive). This is especially notable on Riven and Renekton as they have a lot of aoe and can't make a whole lot of use of the spellvamp in duels. Urgot also enjoys the spellvamp with his spammable Qs :D
Though, Sivir does seem to enjoy something like this, but eh. Most AD Carries would probably go with the regular build anyways.
A really cheap way out is to have a passive that decreases the holder's attack speed but honestly Riot would never do something like that.