Not trying to be pessimistic, but if I had to guess I don't think it'll do much since the PvP clan scene wasn't that great pre-EoC, which is what Legacy looks like. The only way it'll make a true comeback is if they revitalize the wilderness and make it a unique and profitable place worth going to and by fixing other problems.
Right now, I'd say it's doing O.K. with Warbands. The wilderness gets active every 7 hours. If you look at it in another way, that's actually a huge improvement for a very long time. You're GUARANTEED action--even more than Rev Caves--even though that's slightly different. For comparison and by sheer numbers, I don't think Legacy's going to increase the population of the PvP world more than what Warbands has done. I feel like it'll be something there for nostalgia then eventually die out in favor of EoC.
Of course I don't mind Legacy to improve the state of PvP. Just my guess if I had to put money on it.
Some pics:
Hitting a warband camp
(http://gyazo.com/d22b699d05ed55fb8197a68a87e0ffb0.png)
Run-in organized after warband
(http://gyazo.com/d22b699d05ed55fb8197a68a87e0ffb0.png)
The RS3 world was already this active for a while and a lot of us (including myself) were ignorant of it...