First things I'd do?
Find ways to break into new demographics with new distribution models, release schedules, the whole thing. Recruit hard from outside the industry to bring new voices to the table. Then take a hard, long look at the company's long-term options and push as hard as possible to reach that potential. I feel like the industry is too focused on short term gains. With comics selling like they do I understand why, but if you can't plan for the future you'll never have one. And they are way too insular; working in comics is an exclusive club that is really hard to join and it's killing the talent pool.
My goal would be to have the way DC comics are created, shipped, consumed, and viewed totally changed by 2021. I'm talking strong push into digital spheres (not just Comixology), regular in-continuity OGN's pushed in bookstores away from the "spinner racks" and a few other formats that would take too long to get into here, with additions to the digital experience beyond just scanning panels into a computer and putting them into a slideshow. I got tons of ideas for this but I'll keep them to myself since I'm actually working towards that kind of cross-media format with my own project.
I'd change the entire DC webpage. Last time I visited that site it was horrid. I'd make that site the center of the DCU and include fun extras like articles written by Lois Lane, interviews with Booster Gold, live-action reporting done by Amy Adams and whoever it is who plays Linda Park or Iris on the CW shows (I dont watch them anymore so I dunno what's up there), all done in-character of course. As well as exclusive previews and interviews and all manner of junk. Visiting the DC site should be like stepping into the DC Universe itself and I wouldn't be happy until that happens. I might even shut the whole thing down for a couple days with a "site unavailable due to Crisis Event" put on screen to hype a new big story.
(probably not but it'd be a fun gimmick to think about trying).
I'd try to work a deal with the larger media stuff like the movies, cartoons, and games, to advertise the comics during their air time. Man, I'd need to spend so much money those first few years I'd give WB execs high blood pressure, but I could make the DC logo matter again if they let me invest in the IP's and the brand. And that's smart investment; DC's biggest names are so huge and have been such a staple of our culture for so long we're basically born knowing who the Trinity is.
I could write a whole marketing plan on this, but CBR isn't really the place for those details.
From the creative end of things.....I'm the EiC; I dont have time to govern how each single book is written or what direction they go in. But I'd make it a big part of my first year in the position to bring in talent from beyond the usual crowd. Authors like Steven King and JK Rowling, musicians with a love for the IP's and/or medium, unknowns hanging about DeviantArt hoping to break big, actors who can also write (I hear Ed Norton did a lot of re-writes to Incredible Hulk, for example). I'd try to get new, fresh voices on the books and the characters and then hype the hell out of it. These big names from beyond comic circles would bring their own fans to the projects, and hopefully some of those fans would stick around.
The only caveat to that is Nightwing and Superman. Nightwing's an IP with tons of growth potential and one of the highest sales floors in the industry that I know of. I'd ensure that Dick Grayson got his due. And Superman wouldn't be treated like an afterthought anymore either. And I'd just try to hand Wonder Woman over to Patty Jenkins and call it a day.
Honestly, I'd probably piss off all of you. Given how badly things like same-day-digital went over with us hardcore established fans (we all hate change no matter what) I'm not sure if me being in charge of DC on any level is something you guys would want. But I'm pretty sure I could bring in a whole new generation of fans to replace us.