Adding Elbow Articulation
One of the biggest problems with the Marvel Universe line is that most of the female figures to date (relief coming soon?) have no lateral movement at the bicep or elbow. This might help you make use of the figs you have laying around til the new format arrives.
Dismantling the Arm and Gathering Parts
Carefully cut off the molded on hoop where the lower arm is currently attached. Keep the cut along the contour of the upper arm so that you cut only the hoop itself off.
In this pic, I made the cut while the lower arm was still attached, but it's recommended you remove the lower arm first, keeping all parts, as they will be re-used.
Drilling and Adding Joint
Once the lower arms and the hoops are off, then take something like a pin vice and drill out a decent sized hole in the upper arms, where an articulated elbow peg could be inserted.
You will need a donor joint/peg. I used 25A Baroness v1/v2 for this project. The hoop is almost identical to the MU hoop, but it's not ideal because of the peg shape. Not shown in the pic is that you will have to trim down the disk at the top of the peg to be a small pointed ball, just a touch larger than the stem. If you use other joints (star wars knees, or the Uhura knee joint I used on Storm), you might not have to do that part.
Next, attach the lower arm to the new hoop/peg. With the MU joints, you'll need to use the small plastic "bolt" (for lack of a better term) different than with a Joe joint. Here (not shown) you will want to place the bolt through the hoop, THEN put the hoop and bolt into the arm itself, wiggling it into place, before pressing down to secure the joint.
In the pic above, I had not yet trimmed down the disk, so do it now if you have not already done so.
Once trimmed, insert the peg into the upper arm's hole, and you're good to go. You may have to trim more from the peg/disk until it fits, or drill a wider hole in the upper arm, but at some point, you'll get it to fit right.