I'm in favor of anything that attracts more players to PvP. However:
Most players will feel they cannot make a difference in any large-scale format. I'd suggest the problem with FL is not that it's 3 teams, it's that the teams are large. 8v8v8 or 8v8 (probably on smaller/shrunk maps) would address the problem more effectively IMO. I understand some dislike the RNG, particularly on Seal Rock, but equally lucky spawns late are a reason to keep trying precisely because they can swing a match.
Increased variety sounds great in practice. The issue is diversifying options also means diluting each mode. SE bends over backwards to produce short queue times (too much IMO). From their perspective, I think they'd worry a new large-scale mode would have this impact. Of course, if it was good enough, that need not be an issue.
Wouldn't a RW roulette require more than one map? I don't like the idea of simply adding RW to the current FL roulette since I already have to skip one day in three due to Shatter, and given SEs refusal to provide PvP tutorials, many of us simply don't understand RW.