Shared power pools would go a long way in the right direction. But I think power source should also share some common mechanics.
They already do (or did). There are no Martial Implement powers, for instance, and, in the PH, virtually no arcane weapon powers. Divine Implement powers are likely to be a holy symbol. Divine tends toward radiant, Primal towards 'elemental' keywords, Psionic toward psychic, Arcane towards force (and, really, a vast vareity), and Martial towards, again, weapons.
I suppose I should have said
more common mechanics, as yes, I mentioned martial weapon powers and the divine use of radiant damage in this thread:
I think that WotC did an OK (but hardly great) job at giving power sources their own flavor. The prevalence of radiant power in the divine power source is an obvious example, as is the dominance of weapon attack powers in the martial power source. Most of the primal classes have a transformation theme, and psionic classes... umm... well, the divine power source is a good example.
But, you could embrace and deepen those distinctions, as long as you don't cross the line and start gimping or overpowering one source or another.
For example, maybe the martial power source is marked by using stances and tricks instead of at-will powers. And maybe Daily powers are mostly replaced by constant bonuses, a small number of big-hits, some special stances, and "tactics" - daily powers with an extra benefit if initiated before the encounter begins (e.g. a warlord-style strategy that can either be explained ahead of time, or improvised mid-battle).
A perfect example of crossing that line.
Maybe the arcane power source is marked by having more daily powers than usual?
Another fine example.
Really, you're not talking about refining or expanding the use of Sources, you're talking about returning to the imbalances and stereotypes of prior editions.
Oh, come on... I'm not suggesting that these are the
only changes I'd make. Of course it also has to be balanced.
If the classes all have "meatier" class abilities, more daily powers (or access to stronger ones) could be a characteristic of at least some of the arcane classes. Replacing daily powers with static bonuses for martial classes is just describing essentials. Warlord daily powers that provide a greater benefit if you use the plan before combat? That's something you could do today.
When the designers were working on 4e, they didn't start with the grasp on the power system that they have today. As the essentials design shows, the underlying rules mechanics for 4e is capable of generating a set of classes with a more classic D&D feel than they managed in PH1. Power sources are a good medium for this because they represent a collection of common characteristics of the in-game reality, that should be reflected by the rules mechanics.
Martial powers could be less "spell-like" if the designers strictly limit themselves to plausible non-magical effects, and aren't as free-form with player-based narrative control as an explanation. Stances and powers that trigger off basic attacks are another good idea in this direction.
You can also alter arcane powers to get something closer to the old magic-user feel. Maybe more arcane daily powers should have a "disruptable" keyword that means they trigger an opportunity attack from adjacent enemies that -- if it hits -- the daily power is lost? (Obviously, not every arcane class can have this characteristic, at least if we want swordmages to use these powers...)
It's just drawing a tighter connection between the rules mechanics and the in-game reality.
-KS