Personally, I think we should have a well balanced vault, with some emphasis on our strongest synergies. And make sure not to forget new deck possibilities with the newer cards. Fire is the most obvious synergy, but Time has been growing as a potent combo as well. One of our main problems will be good offense since unupgraded Gravity isn't as good as I'd like. Also, permanent control is something necessary to include.
So as for the amount of cards we should pack...
Aether - Medium. Fractals and Quintessences for the most part, with some phase shields.
Air - Low. Flying Weapons and Eagle Eyes will serve us for the most part.
Darkness - Low-Medium. Could make an ultimate denial deck perhaps? Steals are always useful.
Death - Medium-high. A good synergy. Death bonuses plus supreme CC works well.
Earth - High. Pulverizer is good, plus armored Otyughs, Earthquakes and other powerful high HP creatures.
Entropy - Medium. Discord wrecks a lot of stuff, and Black Holes makes it even worse. Novas are always important, and perhaps some Maxwell's?
Fire - High. So much stuff to love, and the synergy is top-notch.
Life - Medium. Adrenaline mostly, with support creatures. Otyugh+Thorn Shield?
Light - Medium-Low. Holy Lights to mess up Dark and Death are nice, but other than that and blessings, there isn't much synergies. Armagio Angels perhaps?
Time - High. Pharaohs and Dune Scorpions strengthen the existing synergy rather well.
Water - Low. Not many good synergies here.