I play a male Shepard hertero so my choices would be from
Emily Wong sadly not developed in ME2
Kasumi Goto a great character and personality
Khalisha Bin Al Jalani? Much more interesting in ME3 and potentially expandable from ME1 though again not an option that was developed
The ME1 undercover operative on Noveria (Parasini) also sadly not developed in ME2 as Parsini was obviously a Shepfan
No Alliance crew members on the Normandy
Odd I seem to be wishing that Shepard developed a Geralt of Rivia personality with regard to liasions with the opposite sex. Certainly Shep could have easily spent time with Parsini, Dr Michele, and Shiala. Can't see my Shep spending a lot of time in the Courtesan's hive in the Presidium
standing around sipping cocktails and small talking. He/she after all is a man/woman of action!
edited replaced Dr Chakwas with Dr Michele. Chakwas is totally great, but a little matriachal, even for someone of my age