"Tobi is Fugaku" theory--->Tobi can be any Uchiha
The main basis for Tobi being Fugaku is the fact that Fugaku was absent during Kurama's attack on Konoha.
First of all, Tobi is way to strong to be Fugaku. Fugaku didn't show anything that put him out above the other Uchihas...in fact, he didn't show anything. I doubt he had S/T techs and a powerful 3-Tomoe if he relied on Itachi so much.
Second, this theory can be used to state that Tobi is any Uchiha clan member, seeing as the whole clan seemed to be absent. So singling out Fugaku is pointless without any support other then the fact that Tobi is helping Sasuke out, which he only is in order to further his plans.
So yeah. Tobi can't be Fugaku, well I highly doubt that he is, seeing that Fugaku showed nothing to put him on Tobi's level, and the fact that this theory is too broad, and it can go for any Uchiha.