For which names could [name]Tess[/name] / [name]Tessa[/name] be a nn besides T(h)eresa or something awful like Contessa? Is there anything?
[name]Tertia[/name]-Latin name meaning “third child”
[name]Terentia[/name]- feminine form of [name]Terence[/name]
[name]Theora[/name]- Greek name meaning "watcher; contemplater’
[name]Thera[/name]- greek name meaning “wild; untamed”
[name]Thea[/name]- Greek name meaning “goddess”
[name]Thirza[/name]- Hebrew name meaning “pleasantness”
Contessa _Italian for “countess”
I really like [name]Tessa[/name] as a nn! Some of these might be a stretch…
[name]Odessa[/name]
[name]Odette[/name]
[name]Treasa[/name]
[name]Talia[/name]
[name]Theodosia[/name]
Tulia
[name]Talisa[/name]
[name]Teresa[/name]
Contessa
[name]Quintessa[/name]
[name]Tressa[/name]
Tesla
[name]Chester[/name]
[name]Celeste[/name]
[name]Estefani[/name]
[name]Esther[/name]
[name]Honesty[/name]
[name]Latesha[/name]
[name]Lester[/name]
[name]Tanesha[/name]
[name]Tempest[/name]
[name]Tennessee[/name]
I love the name [name]Tess[/name] ([name]Tessa[/name], too), and love them as names in and of themselves.
If you’d like it as a nickname, I was thinking that you could always give your baby T.S. initials (such as [name]Tabitha[/name] [name]Scarlett[/name]) as a way of reaching the [name]Tess[/name] or [name]Tessa[/name]. Another possibility could be a T first name paired with an Es middle, such as [name]Estelle[/name] or [name]Esme[/name].
Good luck! 
[name]Thessaly[/name]
This one is really pretty.
Doesn’t this remind you of [name]Cecily[/name] with a lisp?
Doesn’t this remind you of [name]Cecily[/name] with a lisp?[/quote]
No, not for me. My only objection is that it is a modern name (for a person vs a place, that is), and I prefer names with deeper histories.