I would like to make a proposal about summons, what about if we can introduce in somel loot tables, some books in wich are described some additional knowledge about summonings, for example a book for demons one for devils one for elementals and so on.
Then you use this book ( that is to say you read it) and your summoning table changes in base at what book you readed.
So we can have some variety at what we are summoning wich can be nice, imagine to see a villian summoning demons even at low levels for example.