Since you are looking at an Emigre font try out Tarzana, it has an interesting sans look. Dan

So the list grows...

Daniel - I like Tarzana, it has many of the attributes I suggested. I also briefly looked at Triplex, but the italic (which I'll need) is a little too funky.

Tiffany - Gotham is (of course) brilliant, but I feel it's too geometric for this gig. While it would pair nicely with Leviathan, it's personality is not quite right.

I have the follwing in contention at present: Base 9 or 12 Sans, Optima, Tarzana, TheSans.

I'm going to check out some of Nick's offerings at ShinnType as well as Rian H's (Device). More face and foundry suggestions are more than welcome.

Tiffany:

Good suggestion with Gotham. In my mind gothem is a great "alternative" to gill sans (which I am really not overly fond of).

hildebrant.

Take a look at Fedra Sans from typotheque. Also check out Terminal Design's Giacomo and ClearviewOne. Plus, this one from Storm Type Foundry, John Sans.

Wow -- some really great suggestions here. Not only have I been exposed to some fantastic new faces and foundries, but I'm reconsidering the Leviathan pairing all together.

Daniel -- John Sans is BRILLIANT, as are the faces from Terminal. Their quality is immediately evident upon viewing.

I also found a face called Lux Sans at Thirstype which has a nice feel. Feedback on this faces' quality would be appreciated.

So my shortlist is as follows (in no particular order), but funnily I'm still hanging on to Base 12 Sans (my first instinct) because of it's character and small caps as well

Base 12, Emigre
John Sans, Storm
ClearviewOne, Terminal
Giacamo, Terminal
TheSans*, Fontfabrik
Lux Sans, Thirstype

I've ruled out Tarzana and Fedra Sans due to certain idiosyncrasies.

*Is there anywhere online where one can actually set/testdrive Fontfabrik typefaces with one's own text sample? I'm able to do this will all the other faces above at various sites. It's nice to be able to see the same words in context when comparing faces.

-t

I failed to mention Sauna from Undeware whose character also may fit that of my client.

-t

I've ruled out Tarzana and Fedra Sans due to certain idiosyncrasies
You should check the newly released Fedra Sans Alt, which removes some of the idiosyncrasies you are mentioning.

I would use Gotham. Although Leviathan is a gothic, and Gotham is more of a geometric, I really think that Gotham would compliment it. This is breaking my own rules, but you only live once. And besides, they are both by Hoefler Type Foundry so how could you go wrong?

*Is there anywhere online where one can actually set/testdrive Fontfabrik typefaces with one's own text sample? I'm able to do this will all the other faces above at various sites. It's nice to be able to see the same words in context when comparing faces.

Tim, try fontshop.com