Ughhh... I think Harlies are over-rated. It seems that you have your HQ and Troops settled out. Now, time for at least one unit from each of the other FOC options.
Elite - Actually, any of the Elite choices are a good bet. Harlies cannot take a transport, but as they work well in small squads, they can always hitch a ride in a Falcon. My preference, I suppose, is Banshees. I really like not needing to worry about striking last when assaulting into cover with these fragile ladies. Plus, the 30 power weapon attacks are nice, too. Scorpions take a close second, while Dragons are pretty cool, and I do have a squad, but I find that I would rather see the ladies chase all the 'manly' Marines around.
Fast Attack - Here you have some options that look great to some, and terrible to others. I like Vypers, especially when added to an army themed around speed. For infantry armies, Hawks and Spiders are awesome, and can do just about anything Vypers can do for about the same number of points. For some combat punch, think about Spears. They work well in just about any army setup, but you will be paying for those fast Str6 power weapon attacks. Just keep in mind that like all Eldar, even the jetbike specialists are a bit fragile.
Heavy Support - This is often the area where armies are defined. Do you want to include only infantry? If so, the combination of Wraithlords and Reapers will make anyone from Guardsmen to tank commanders concerned for their own welfare. Prisms and Falcons are pretty good, solid choices, especially for competitive play. These units are not always very effective at removing enemy models from the table, though it is fairly rare that they give away victory points. The transport ability of the Falcon makes it a near-ubiquitous favorite. War Walkers have a place in many players' hearts simply for the sheer massive amount of firepower they can dish out, especially when guided by a Farseer. Though it must be said that of all of the Heavy Support options, War Walkers are the most fragile, so must be used carefully.
If the player in question is somewhat new to the game/army... Try...
Scorpions
Hawks
Wraithlord
These options will allow for more experience with many of the game/scenario rules that the current HQ/Troops do not permit. Hopefully, these options will complement the models already owned and will make for more interesting and fun games.