Yeah, Legacy would need some serious juicing up, specially if you plan to monotize it, you want people to go crazy for the stuffs that's in there, and most of it is kind of meh.
Solution #1: For every level of legacy you have, you get that % off the cost of account-wide Legacy perks in in-game currency. Level 50? Enjoy having all of those perks and unlocks cost 50% less.
#2: 10 cartel coins per legacy level, every month, if you're subbed. This isn't game-breaking in F2P, it rewards subs and veteran players.
#3: Every time you start a new character, for every 10 levels of legacy, you get to buy one character-specific unlock for free. Keeps those of us at Legacy 50 still rolling new alts.