Tokyo Game Show, A.K.A TGS is held September 24-27 this year.
So that's a no go if the thread starter even wanted to go there.
I cannot offer you any advice regarding what the scenery will look like.
But regarding festivals, the question really is: how much time are you willing to spend at festivals ? Sounds like with all the temples,shrines,sightseeing,hiking, cycling etc. you have yourself a packed itinerary already.
If you reckon that you'll go for more than 1 or 2 festivals and you want to see as much as possible than yes pro-longing till the summer-ish area is a good idea.
But it sounds to me like you haven't checked what existing festivals there are in December. Take a look at your options:
Japanese Festivals | December :: Festivals in Japan
There are about 15-20 festivals in December already; seems like enough to me.