Most new poker sites know that they will have to offer some incredible bonuses and promotions in order to draw customers away from their "regular" poker sites. It would be expected that Harrahs WSOP Online would do the same thing.
How exactly they would accomplish this, or what sorts of bonuses they will offer remains to be seen. They will likely have some sort of first deposit bonus to draw customers in. All of the major rooms offer this (although PokerStars' bonus is very small) so it would be expected. To keep customers they may offer reload bonuses and other promotions. They can use a points system to reward loyal customers and may even tie that in to the Harrahs Total Rewards system already in place at their resorts and casinos around the country.
They will likely have some sort of rakeback system set up as well. Many new sites offer ridiculous rakeback rates, although Harrahs WSOP Online may not be forced to offer as much as some of the smaller rooms have done. Rakeback has pretty much just become part of the industry and it would be difficult for them to compete with sites that do offer it if they chose not to. That is unless their rewards program is just so incredible that players could earn some of their rake back in other ways.
Either way there is little doubt that when Harrahs WSOP Online hits the market they will bring with them some sort of desirable combination of bonuses and promotions to entice players to make the switch.