All right, I am in a time crunch, but I do feel that I want to contribute to this thread. I want to help the OP. Sorry for grammar and spelling in advance--I'm crunched for time doing something else. Here are some ideas which I am pursuing to make myself more attractive to a legacy. It's not easy, but the reality is that we all have to make ourselves stand out; only hard work can get us where we wish to end up.
1. Obtain another type rating. Transition over to a different airframe if your airline's fleet possesses multiple types. If you're at Republic, this is impossible. Look for part 91 contract work. These gigs are out there, and they will type you if you seem to bear potential for their operation. Sign up for part time sim instructor work at CAE Simuflite or Flight Safety International--it will look good on your resume and they will also type you in whatever it is that you teach. Another option: go to Spirit, Frontier, Allegiant, Atlas or whoever else will type you in a bigger turbojet/fan aircraft. More long-term potential for earnings, 401k and Delta for example's sake has an obsession with pilots showing initiative to advance their careers.
2. Enroll in an online program to get a master's degree. I am doing one right now from FIT for $18k. They are time consuming, and some are aviation related, and will open the door for you with the FAA, NTSB as well as safety departments at your airline, giving you a headstart for a position with them which will likely make you very attractive to a legacy carrier.
3. Volunteer--do this as much as you can! It doesn't have to be every day, just do it two hours every two weeks if you must. Become involved with multiple programs. Seek out WAI, OBAP, NGPA for volunteer opportunities. Look for volunteer opportunities with ALPA. I just found out last week that UAL ALPA's headquarters are located five minutes away from where I live, and constantly seeking volunteers. Good opportunitity to volunteer, network and make the resume shine. I have been living where I am now for a year and feel stupid for not researching this earlier. EAA, AOPA and aviation universities are other ways for you to get involved with aviation groups in your community. You would be surprised how many EAA chapters are near you!
4. Keep checking off as many of those boxes as you can on airlineapps/pilot credentials. Get a seaplane rating, glider rating, CFI certificate and don't stop there. Check off every damn box to show them that you are not a stagnate pilot!
5. Acquire an official position with your company doing just about anything. Professional standards, union representative, safety department, hotel committee--it doesn't matter. Just do it!
6. Network! Go out where mainline pilots gather, talk to them, make connections, offer to become involved with whatever presents itself and have them refer you.