I would like another item too, but I can see form a play style standpoint as to why there is not. Almost every AP champion is burst and/or combo based or DoT based (Swain and Cassiopeia for example). Where most AD is either continuous (such as in AA based marksmen and fighters) or burst after a delay (like Zed's ult for example).
is a great passive to counter burst. The shield can prevent most combos from 1 shotting while the amount of MR can soften up the poke damage to a somewhat more manageable rate.
is great vs combo burst champions who have to all in to kill (mainly assassins). The active makes any combo less effective, if not completely nulled. The armor also weakens the poke from abilities. And for just 1100g, the 45 armor and 35 AP from just 
is not a bad deal, plus it has no cooldowns to deal with, unlike .
The reason they are set up like this is because most AP champions either out range or can out maneuver most AD champions. Champions like Lux, LeBlanc, or Xerath should have no issue with harassing a squishy immobile marksmen from range or with their mobility. So the amount of incoming trade damage should be small. 45 armor should be enough.
Where as an immobile and squishy AD would have minimal opportunity to win the trade/poke war, let alone the instant kill threat. The shield prevents the threat of instant death every so often and the MR staves off some of the chip damage.
In this case I would say Zhonyas is actually preferred. It does everything that is needed for a burst AP champion to be effective and also counters the burst assassins, as well as stops a lot of abilities from working properly, as well as stalls for your abilities to come off of CD.
Now, there are those champion which do not follow the "rules" and make the items work better than intended. These are AD fighters. Champions with enough natural defensive stats to make them not squishy, and with good enough AD stats to make them a damage threat, along with mobility and/or massive sustain/shielding. The number 1 offender is obviously 
followed by currently 
.
Sad thing is that there is not an easy way to introduce an item to counter only these champions without benefiting the counter to others more so than they need. For example, lets say a new item is made that gives a good amount of AP (like 70 or so) and also mitigates AD damage taken by the same champion by 20% for 5 seconds (think adaptive helm passive). This would prevent fighters from dealing that crazy repeated damage, but also would inadvertently nerf all marksmen severely.
tl;dr
I agree it would be nice, but it probably could be very hard to make because of how it could significantly hurt other champions who are not part of the issue.
who are all allowed to sit on 
, but there is nothing to match 
, 
and an early 
unless they're getting shit on. It's the same the other way around. would why I skip 
on a mage like 
or 
unless I'm getting fucked up by an assassin and need to rush 
or any combo of items that bring much more to the table than 50AD and 50MR like 
+ 
or 
+ 
...
The problem here is that mage players would rather go a static core build then complain when they get shit on rather than counter-building like you're supposed to be doing every game. And there is no arguement that mage's don't have plenty defensive options. 
is the closest thing available, but not every champion has synergies with spellblade.
+ 
+ 
+ 
+ 
