I think merging everything completely into a single forum is just going to create too much noise. By having more than two dozen different brands with little in common aside from a points program, will make it harder to sort through the volumes of posts for the info relevant too you. If the whole point of FT forums was to discuss the workings of the loyalty program, than a single forum can work. But FT discussions go way beyond the program, and even down to reviews and info about specific properties. Most of the FT discussions seem to revolve around the brands and products (and complaints about how the run their IT). For that reason, I think segmentation is useful.
That said, a Fairmont-specific forum might be too specific. Another possibility might be a subform for all of the former FRHI brands (two of which never had a forum on FT at all), plus Sofitel, M-Gallery and Banyan Tree, and you create a collection of brands that might have some appeal to the same profile of traveller. I'm sure I'm not the only traveller who would never go in a Novotel, much less and ibis, so a lot of the traffic on a monster forum would not be relevant.
If the only options are 1, 2 and 3 above, then I like
#3 . But, I also like the idea of dividing the brands into perhaps 3 different quality tiers to make following and searching discussions much easier.