I've been wanting to do this with some of my older ProgPower shirts since I was getting them in large back then. Even some of the newer ones that I got in medium aren't very feminine so I've been thinking about doing something with all of them. One thing for sure is something needs to be done with the sleeves.
What I want to do is take a little off the bottom and a little off each sleeve. For the sleeve cut, I'd angle it more dramatically towards the underarm to make something similar to a cap sleeve. It will also blend better with the new seam I want to make, and remove some of that excess fabric that girls don't need ;-) Cut a deeper neckline like you mentioned. Cut a more curvy pattern that will become the outer seam on both sides, so it looks more like this )_( instead of this |_|. That will make it a more flattering fit, and also make it a size smaller. Then sew the front and back together with contrasting colors, just for a little DIY punk-rock feel. For a black shirt, try hot pink and lime green, or red and white. For white shirts, try black and purple. Sew it all the way up the side and under the sleeve in one smooth seam.
Then cut a deeper neckline in the back also, or just cut small horizontal slits about 1 or 2 inches apart across the back (not all the way! just fold the shirt in half and make some snips). When the fabric stretches, it will look like cool rips/strings. With some of the scraps from the sleeves or bottom, cut two long strips and tie one around each sleeve (through the neck hole and sleeve so it makes a bow on the shoulders and pinches the sleeve). It will give the shirt a bit of a girly feel but also be interesting. For a fun touch, instead of scraps from the shirt, use scraps of a totally different color or texture for the ties, like plaid, or something shiny.
The bottom of the shirt can be trimmed with lace, fringe, or more of that same scrap you used for the sleeves. Or you can cut vertical slits along the bottom, every one or two inches, for a make-shift fringe.
There is a group on youtube that I've gotten a lot of ideas from. They are called THREADBANGER. You'll find hundreds of projects you could do, and some even talk about clothing customizing.
I can't wait to see what you come up with! I have all kinds of ideas, but lack the sewing skills. Post pictures if you decide to make something!
I guess I should have checked them out this year. I didn't even look at them cause I figured none would fit. Then my friend Pam shows up with an awesome Sabaton shirt last Saturday night that I'm sure would have fit me!!! 
