Mainly improve soundstage
I would start with non-destructive mods. Often you can widen the soundstage by swapping the pads to something else (like oval HM5 pads perhaps). Or deepen the pads by lining the inside of the pad with kleenex, caulking backer rod, cotton balls, a piece of coax cable, etc. At a minimum I would add some dampening to that cup to reduce reflections - adhesive closed cell foam, sorbothane, dynamat, etc.
If that doesn't get you the changes you are looking for, you could try to put a few holes in the side of the cup, at the bottom (like this headphone I modded):
Start with 1 hole, then go to 2, then 3, etc. Or you can start with small holes, and slowly enlarge them. This way, you can attempt tune it with the number of holes. If you have any problems, you can fill the holes if necessary (or cover them with tape). You can also tweak the holes by covering them (from the inside) with a strip of micropore or transpore tape (in effect making the holes smaller by increasing resistance).
Also note that as you increase soundstage, bass may increase as a side effect (or it may not). If it does, or how much, depends on the individual driver, cup design, cup volume, etc. And obviously making the headphone semi-open will affect isolation.
Basically when you mod a headphone that hasn't been modded before, recognize that you are totally experimenting and being a pioneer. You are going against tuning that was likely done by a whole team of professionals. That doesn't mean you can't make it better (or different). You may make positive changes, or you may make negative changes. You may get the results you are looking for, or you may get terrible results. You may improve midrange, but make the bass tubby at the same time. Or you may make the bass tighter and cleaner, but the treble thin and flat at the same time. You are basically being a mad scientist in a way. That's why I recommend starting with mods that are small and hidden if possible. This way, you can cover up your mistakes if you make any. Or, if you have a headphone that you hate (or was really inexpensive) and you could care less if anything bad happens to it, be bold and just go for whatever you feel like.
Be aware that in many cases, it's easier/cheaper just to buy the open version of a can (if one exists). For example, if makes little sense for me to try and mod the closed soundMAGIC HP150 to get more soundstage (by making them open), because an open (and properly tuned) version already exists (ie the soundMAGIC HP200). I don't know anything about the AKG model you have (and really don't know much about the AKG line in general), so I can't really help from that aspect.
If you do decide to mod, always make sure the vent holes on the driver are taped off while you are making holes, and clean up all dust and shavings. Don't forget to remove your tape when you are done. I also recommend doing the work with the cups apart, so you can make sure you don't damage the wiring and/or driver. It's easier to do the work with the pads off and cups taken apart. Depending on what I'm doing, I will also desolder the drivers and get the, totally out of the way. But I understand that is too hard for some folks, so at least open the cups and keep the drivers out of the way. The wiring is short though, so you don't have a lot of room to work with - be very aware of this or you risk ripping the wires right of the drivers.
Finally, note that with only 1 pair of headphones (and I am assuming no measurement equipment), you are going to be relying solely on sound memory for your changes. Sound memory is not very reliable, especially when there is a gap in time between listening sessions while you are physically performing the mods (drilling holes, taping this, assembling and disassembling, etc). It is much better (yet not as pratical or cost effective) to have 2 models of the same headphone, so you can A/B your changes.
Good luck and most of all have fun! Use this as a learning experience. Be sure to post back with your results!
