Other than [name_f]Harriet[/name_f] or [name_f]Henrietta[/name_f], what could get me to the nickname [name_f]Hattie[/name_f]?
The only [name_f]Hattie[/name_f] I’ve ever met was just [name_f]Hattie[/name_f] - no full name! I personally think it’s one of the few nicknamey names that really stands on its own.
Other options:
[name_m]Hart[/name_m]
[name_u]Hartley[/name_u]
[name_u]Hadley[/name_u]
[name_f]Hester[/name_f]
[name_f]Hestia[/name_f]
There’s always [name_f]Hadassah[/name_f] with soundalike [name_f]Haddie[/name_f] as a nickname!
If you aren’t set on a middle name you could combine a Ha name with a T middle? I have created some random combinations:
For example:
[name_f]Hannah[/name_f] [name_f]Therese[/name_f]
[name_f]Hallie[/name_f] [name_f]Theodora[/name_f]
[name_u]Hadley[/name_u] [name_f]Tamzin[/name_f]
[name_u]Halsey[/name_u] [name_f]Tulip[/name_f]
[name_u]Harlow[/name_u] [name_f]Tabitha[/name_f]
[name_u]Harbor[/name_u] [name_f]Topaz[/name_f]
[name_u]Harper[/name_u] [name_f]Thalia[/name_f]
[name_u]Harmony[/name_u] [name_u]Tate[/name_u]
Henriette
[name_f]Mehitabel[/name_f]
[name_f]Hortense[/name_f]
Haleakala
Hatsepsut (sp?)
You could just use Hattie 
Otherwise
Charlotte
Hazel Ottilie
Hannah Matilda