The problem is that as long as the item offers stats that directly contribute to burst, your guaranteed to have the AP assassins abuse it, Mages arent the problem, the AP Assassins are, and limiting it to melee only removes
,
and
from the pool of abusers, which still have

also two problematic AP champions that are not Assassins are
and
who are infamous for supposedly being "Auto Wins post lv16", which means that the Item cant have AS, so that it wouldnt fuel
and
in terms of Absurd DPS (also to avoid
,
and
from abusing it)
so with that in mind, what can we give the Item, that wouldnt Appeal to the Assassins as well as Burst Mages (
) and Artillery Mages (
) but would appeal to the AP Bruisers and the Battle Mages (The other AP class that was meant to be the more durable subclass, the Mage's counterpart to the Fighter's Juggernaut)
well let us look at what they all have in common
AP Bruisers

Battle Mages

they all have some way of dealing Damage over time, whether an official DoT effect, a Drain, low cooldowns that allow spamming, abilities with charges, they also have some form of survival such as Healing, a Resurrection passive, disengages, shields, high health pools, Hard and Soft CC
and finally they all share a low base movement speed and being very kite-able once their CC is on cooldown
Items that most of these champions buy include

Items which grant HP and AP, stats which both groups like, but why do we see morello taken on other AP classes too but not the others?
if we look at the others, we can see that they offer some more defensive power (Slows, MR, Awe Passive) or more DPS power (%HP Burn, MR Debuff Aura) in exchange for some raw AP
by that logic, the most optimal stats and effects to give these items is Health, a low but not Irrelevant amount of AP, and then one or MAYBE 2 of the following: Passives or actives that allow for more DPS potential or survivability, some defensive stats like Armor and MR, CDR, Mana or Mana Regen
Alternatively you could give AP Bruisers and Battle Mages HP Scalings like how AD Bruisers have AD and HP Scalings, this opens up items like
for them, items which AD Bruisers start to buy after a couple of AD Items