Should a succession by Princess Taiping even be in the equation? What about skipping over that and having Wu Sansi or Wu Chengsi as heir instead?
I feel it would be very difficult having Empress Wu establish a real dynasty unless she had a male non-Li heir somewhere out there. I think the best she can hope for is something like Empress Wang and Wang Mang: if she declares her own dynasty and has a nephew or something become Emperor instead, then she can solve the heir problem. I know Wang Mang came to power in a dynasty that was already decaying and after Emperors that had burnt out the goodwill attached to their dynasty though.
Well, as decaying empires go the Western Han wasn't really in that bad a shape when Wang Mang took it over, being really only afflicted with a severe case of court politics.
Princess Taiping is not ideal, but it's really Wu Zetian's only shot because Taiping is still, however unorthodox her succession, a 'Li'. Having the Wu clan (nephew or not) as heir would
really be usurpation in the eyes of the Tang bureaucracy, as even if you hated Wu Zetian before, you at least have the comfort of knowing that the old woman was going to die before long and that her male heirs would quickly reject her legacy. There's also the fact that legitimacy through lineage was actually quite important in Tang society, as demonstrated by the number of Tang epitaphs that make reference to glorious deeds of ancestors (which doesn't happen in the Song).
I think the 'silent coup' that ultimately overthrew her in 705 would have progressed at an even faster rate had she openly declared a non-Li as her heir, which is sort of why she delayed making one.
It is difficult for Wu Zetian to establish a real dynasty, but she is helped by the fact that Zhongzong and Ruizong were pretty average, while Princess Taiping - really the only legitimate heir who has a reason to continue with this Zhou Dynasty business - was comparatively talented. Then again, the necessary support for a Zhou Dynasty would have to be provided by the Wu clan, which wasn't really up to the task.
It is possible to outmaneuver the Tang bureaucracy, which while formidable is of course plagued with issues of its own that a good politician can exploit to his/her advantage.