Mark me down as another one who thinks level-linked spell slots will be a thing of the past. Instead, spellcasters will just have a small number of generic spell slots which can be filled with any spell they know. The main limitation, then (as in Bo9S) will be on the number of spells of each level that the spellcaster knows.
With respect to how spells improve with caster level, I can see two possibilities. First (as in Bo9S), there could be various versions of the same spell, and a spellcaster will have the option to replace his spells known as he gains levels. So, at 5th level, a spellcaster might be able to learn a lesser fireball spell that deals 5d6 damage, and upgrade it at 10th level for a fireball spell that deals 10d6 damage. The other possibility is to simply have the spell scale indefinitely, so that a fireball from a 25th level spellcaster deals 25d6 damage. My preference is for the second option, as it would cut down on spells that are simply numerically improved versions of each other.