I like the idea of a new alternate zone for 55 to at least 70 since that is the latest elite skill option. Obviously this would benefit the players quite a bit. Making a case that it would improve investment into the game is a bit harder to sell. From the game history we do know that CoO/Xav. was added early on and that it was overall well received by the community. Whether that translated into increased sales, only someone with access to that sales info. would be able to really tell.
Other options I seem to remember being suggested at one point or another would include: advanced honor parties for leveling toons past level 30, increasing the xp/tp from instance mobs and bosses, increasing the xp/tp in battlegrounds and of course as previously mentioned above, offering zone repeat options which since it could be monetized seems like it would be the easiest to sell to the decision makers.
All that said, not for a minute am I gonna hold my breath that any of this will ever happen. There really is no evidence to support that GF/RW takes player suggestions all that seriously for whatever the reason(s). Yes, some things may have changed that players have asked for, but correlation does not imply causation. I have yet to see a forum post acknowledging that player suggestions have lead to any specific changes. Of course I may simply have missed that and will be the first to admit I was wrong if proven otherwise.