I think there shouldn't be any cap at all.
While the campaign could go to 20, or whatever other level, it will be much easier for Modders to adapt to an infinite system, than a capped one, which will later have a higher cap (as many cried over the fact that NWN 1 went 40).
Of course, the modders must be allowed to set a cap for their modules, but as with anything else, the engine should be with as few limits as possible (so everything is possible to thecrazy mind).
Even though the official team doesn't "support" epic play in the sense of the word, they don't have to, they just have to open the possibility to others. They don't need to create a single epic monster, just provide the SYSTEMS (class progressions, feats, spells etc.) from Epic handbook.
The second argument for an infinite scaling, if not as good, is that D&D supports it. Acording to the current rules, there is NO maximum level.
I doubt anyone can convince me to think otherwise, but please try B)
