Feat progression? Hit Dice? Base Proficiencies? A convenient way to say "you can't fully multiclass between Wizard and Psion?" That's all that I can think of.
To me these don't sound like reasonable game design targets. Why would the designers want two characters to have the same feats progression? Should we merge all d10 classes into one?
Feat progressions by the way they are an aftermath. The designers are not really wanting one class to get 4 feats and another to get 5, the resulting progressions are such because first they set the key class features (e.g. spellcasting progression), then they add some cool unique single-level features, and only after doing this they look at the results and think "hey the Paladin needs a little boost, let's give her 1-2 more feats" or "there's a blank level here and we're out of good ideas, so feat!".
Hit dice and base proficiencies don't even have to be the same for different arcane casters. They can be, but don't have to. In 3e there were groups house ruling the Sorcerer to d6 HD because they pictured them being tougher that wizards. The designers instead opted for giving them more proficiencies, for some reason.
I'm not saying that these should be different, but neither they should be the same. There is no "should". Framing them all under the same class, freezes some design options. OTOH I have the feeling that there will be Mage subclasses granting a larger HD or more proficiencies (other classes subclasses do that).
Round and around, it always goes back to the original point, that making a single arcane class only to use subclasses to differentiate is just pointless. It doesn't change anything. I can live with this, but it yields this squinted perspective of having a giant Mage class and 9 narrow classes. At which point, I almost hope they remove the 8 schools, leave just a couple of them, so that the Mage class shrinks down a bit.