An arcane-only world would probably feel a lot like Eberron–you’d need to explore the consequences of plentiful access to commonplace magic use. If magic is an inherent power for every individual, on one level or another, how does that flavor the way they go about their daily lives? Remember too that the PCs are exceptional. Even in a regular campaign, not every rank and file soldier is a Fighter. In a full-arcane world, perhaps war is conducted by dueling champions, with no large-scale battles. Perhaps both sides create armies of arcane creatures, from undead to animated soldiers to warforged, and do battle by proxy. Perhaps the majority of people have enough magic to perform a few simple tricks, but still pick up pikes and spears to defend their farms when the threats come out.

There are a lot of ways you can explore the single-source campaign world, really, but ultimately it comes down to what you and your players feel is the most interesting. If it seems unweildy to create a setting where no other power source exists, then the campaign restriction could be something as simple as the players all having trained at the same arcane college, or being part of the same society of arcane scholars–basically shifting the reason for the restriction from a world-level one to something that ties the party together.

Hybrid and multiclass chracters open up lots of options. Perhaps a character who discovered a new aptitude later in life? Or one who trained in magic but found out he didn’t have much talent at it. Perhaps a rogue who has been masquerading as a different class and then finds themselves thrust in among people who know the difference.

For a higher level game, paragon multiclassing is a good way to show somebody who’s switched paths along the way. Personally, I let somebody who’s done that choose powers from either class from then on (it helps balance a somewhat limited choice).

I would consider actually working on the “allied” aspect of the party. All of them use a single power source? Maybe they get a bonus for that. Encouraging Guild or Tribe feats is also a really good idea.