Well, IMHO, I belive Summoners are incredibly strong. However, not seen by many experience parties throughout the trail of 1-75. Summoners are a tad broken in some cases, however, it's fine.
As for new summons, yeah, there's a high possibility. Square-Enix will try and think of a way to make their customers happy. Who says Odin or Alexander will never be Avatars in the Final Fantasy XI world which is, mind you, a different world then previous Final Fantasy games or myths that were told years ago.
They could change the size, shape, and other things to make it equal to the Final Fantasy XI world. Shiva, Ifrit, Ramuh, and other avatars didn't have the exact same size and shape in each Final Fantasy game they were featured in did they? They were similar though.
As for the avatars elements, who says they can't make an avatar of the same element? There is no restriction bounding it to a certain pattern. And as for Fenrir and Carbuncle, they arn't the avatars which represent the elements. There's 2 known groups in the Final Fantasy XI world:
#1 - The Terrestrial Avatars: Carbuncle (Rainbow), Fenrir (Moon), Diabolos (Dreams), Bahamut (Sky), and Phoenix (Life).
#2 - The Celestial Avatars: Titan (Earth), Ifrit (Flames), Shiva (Ice), Odin/Alexander (Light/Dark), Garuda (Wind), Ramuh (Thunder/Lightning), and Leviathan (Water).
And there could also be other groups of avatars, but there's no possible way we can find out. But don't say it's impossible for a new avatar, even though it doesn't matter really, just more or less a way to make summoners and some other people happy, or impossible for a new job(s).