Especially right now, you need to attract candidates who are "passively looking": people who have a job, but would move given a good offer. I do freelance development and consulting, and I'm basically happy with that. I wouldn't turn down a great opportunity, but I'm not going to start the application process unless I know you can beat what I'm already earning.
I'm only speaking from anecdotes but I feel like finding a way to cater to the passively looking engineering job area could be HUGE if done well. Trouble is I don't entirely know how to do it well and not come off as spammy.
Isn't that what SO does, quite well?
An idea I had that I have never implemented but would happily subscribe to is a weekly or monthly newsletter with jard data about salaries. Not self reported salaries, or salaries from job posts, but honest to goodness salaries, anonymized.
I don't know how you would get the data, but I doubt you'd have trouble attracting subscribers and attention if you had good data.