Hey guys,
I'm on the lookout for some sort of VST effect that will allow me to do panning on stereo tracks (busses mostly) similarly to EQing - a line that can be dragged up and down to boost or lower the volume of a track at certain panning angles (as opposed to frequencies like with EQ). 
 
Does this exist? Or am I making it up and just created a new concept that we should have had for years? Or would something like this even be necessary when we already have volume, panning, EQ and compression?

Look up Pancake, a free VST that may be what you're looking for.

Are you talking about a bus that has multiple instruments or tracks routed to it and you want to pan elements in the Bus?  If that is the case then there are some tools but I am not quite sure what you are asking?
 
Melda has a panner that allows you to pan certain frequencies different from other frequencies.  We used to/still do, have Roland R-Mix that is kind of a frequency-stereo location tool for adjusting volumes in regions defined by a frequency-stereo location box.  Of course the easiest thing to do is to pan your individual tracks.

Waves S-1 Stereo Imager might do it using asymmetry possibly

You can use something like this.
 

 
 
Using this you can pan by frequency. If you just want to move the line up and down to do simple panning of course the works, but its no different than using a pan knob.

I get what you're asking for (I think). You want to take a sound that's spread out over the stereo field, and do things like "boost the volume of whatever's sitting at 30% left" and stuff like that. Using an interface that's like an EQ, but instead of the horizontal axis representing frequency, it represents pan position. 
 
I'm not clever enough to work out if that's possible, but when my brain tries to wrap my head around it I get little voices saying "that's probably more complicated to implement than it sounds." I don't quite know why, and perhaps someone is going to reply with "why, that's totally doable, in fact you can do it with X, Y or Z plugin," but what I'm thinking is that if you could do something like that, then it would be possible to do stuff like increase the level of a piano in a stereo mix without affecting anything else (provided the piano had its own definite space in the stereo field). And I don't think we can do that at this point. Unless I'm wrong (or have misinterpreted your question entirely). 
 
 

waves s-1.
 
spread, and tilt

batsbrew
waves s-1.
 
spread, and tilt

I don't think those do what he was asking - spread is just widening or narrowing, and tilt is basically taking the whole field and "rotating" it left or right. I think what Dave is asking is would it be possible to boost or attenuate specific pan positions in the stereo field without affecting the rest of the field. 

I admit I have read the OP four times now and still do not understand the question. Is this a static effect or dynamically based on volume? Due to a stereo field you are going to get a "net effect" rather than say notch out 6dB every 15 degrees in a full circle. You are still limited by the number of speakers you (or your listener) has.

It is an interesting question.  What I am about to talk about may or not be a solution. But they are interesting.  Ben put me onto Waves Brauer Motion plugin.
 
https://www.waves.com/plugins/brauer-motion?km_source=google&km_medium=cpc&km_term=44217550980&km_campaign=828249049&km_device=c&km_matchtype=e&km_keyword=waves%20brauer%20motion&km_adid=201059467382&km_account=searchbrand&awid=1146922547&gclid=EAIaIQobChMIkKqigaCF2QIViYmPCh19hg9EEAAYASAAEgLBPvD_BwE
 
There are a new wave of plugins that are doing very interesting stuff with panning movements.  This one has two panners and they can act on L and R channels of course but also on a summed mono signal too.  They can pan in sync with each other or free run.  In sync with the session of course as well.  Not only that but the front to back movement can also be created as well as the left right movement creating a more circular motion all the way around.  Both panners can be circular or one can be flat left right panner.  There are many adjustments to the panning movements such as how deep and what shape the panning movements are making etc.. from a tight ellipse to a big wide circle.  All variable.  To a flat left right pan back and forth etc..
 
When you get into something like this too you realise how totally boring a flat left right pan actually is because that is what it is.  And that is what most DAW's will allow.
 
You can automate all this Brauer Motion movement too of course and create the very graphs for these panners yourself.  If you feel so inclined.  I thought this might all be a bit silly but once I started getting into it I realised how totally awesome an effect like this is.  It can do stuff to very wide stereo like synth sounds that nothing else can.  You can insert these panners on tracks, buses and your main mix if you want.  Waves had this on sale for $29 and for a time it was well worth it.
 
Another plug I found is called Panagement.
 
https://www.auburnsounds.com/products/Panagement.html
 
This one works in conjunction with one of your fave reverbs, preferably a very nice one.  It allows for very accurate front to back placement as well as left right placement.  There is a free version of this and that certainly shows off the basic effect.  All of these parameters can be automated as well in Studio One.  There is an LFO also for some automatic panning movement.  I have spent some time with this and this too is quite interesting and rather remarkable.  It is a more static type effect though compared to Brauer Motion which can be set up for some head spinning effects!