I chose a serif font because they tend to have a certain gravity and engender trust, which is why we see a lot of serif fonts in the legal profession. I felt these were attributes that would resonate with the clients you describe in your brief. The reason I chose this particular serif is because you mentioned your clients are youth-oriented and do not see themselves as ageing. The style of this serif is quite modern and quirky. The image dimensions probably make the detail difficult to discern, but the serifs are asymmetrical both in size and style. This is why I thought this new font would be better suited to the brief than, for example, Chapaza, which has a more traditional Baskerville-esque feel.
Also, it's unique to this project. I have several fonts that are works-in-progress, but this is one of only two that have never been used before.
As for other fonts - I like, wherever possible, to give clients unique (or, almost-unique) type solutions when they ask for lettermark logos. I have an unpublished three-weight sans serif font family (in the Gotham/Proxima Nova style) which I can experiment with. I have only used it on two or three logos so far, so it remains relatively exclusive. I also released Plateia Bold about a year ago (
http://www.dafont.com/plateia.font), but only one weight. I actually drew six weights, three of which have never been used. Does this 'squarer' look appeal?
The other alternative is to create a lettermark from scratch, which I've no objections to attempting if you have the time to give me some more detailed thoughts about the sort of style that will appeal.