True, we did not include many Frankish (western) magi. There are only a tiny handful, and a fallen Frankish covenant. You could certainly add more - perhaps organised groups of magi from the west arrived in the wake of the Fourth Crusade - which would play up the "culture clash" dynamic. Perhaps this mass imigration was a deliberate plot instigated by the evil Tremere... 
It would be interesting to have a saga where the PCs are Latin immigrants to Thebes, and where the local language and customs seem alien to them. Here you could certainly add a darker tone to such local magi, and play up the "righteousness" of the Tremere ambition to cleanse the Tribunal of such pagan/obscure practices. In retrospect it might have been good to mention a few different saga possibilities like this in the book.
Hmm, I do not entirely agree. I think the Tremere chapter allows room for different interpretations. It is directly stated that Tremere magi desire dominance and that this tends to alienate other magi. Add the rather extreme militaristic tendencies and the system of control exercised by senior Tremere, and you have, it seems to me, a House with an image problem! I certainly get the impression that the Tremere see themselves just as you describe, however I think the Order at large does not tend to share this view.
I agree, I think we are due a Tribunal (outside Transylvania) where the Tremere are doing very well for themselves. Although I would again note that (e.g.) in the Rhine, the Tremere are mostly regular and accepted.
Still, there are only 51 Tremere magi in total outside of Transylvania - that's an average of just over 4 per Tribunal! This means that a Tribunal with an anaemic Tremere presence (e.g. as in Normandy) is going to be the most typical case. Both the Rhine and Thebes actually have an above-average number of Tremere magi.