kitsune9
Adventurer
I don't think levels are necessary, but advancements in some form are.
Items can satisfy this, but the goal is probably not just to get a higher plus - you will want new options that you can combine with older ones, or that improves what an older one did. A Boot of Speed turns into a Boot of Teleporation, a Flaming Longsword can also be used to shoot Fireballs, stuff like that. (A Longsword turning from +2 to +3 is also an improvement, but it doesn't add a new option. You just hit better.)
Levels (especially levels and classes) manage to achieve that. You don't just get a +1 to hit, you also learn a new trick that fits to the ones you already know, either in theme or in effect.
(Flaming Longsword: If it gets the ability to shoot fireballs at individual targets, it fits thematically - it is still all about fire. If you get the ability to make to blindingly fast attacks with it, there is no thematic link, but the effect is similar to the single attacks you already do.)
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I suppose (unique, character specific) powers should be around 4-8. Some people will still "need" more. But you might also want a few options that are available for everyone. (Stuff like Action Points, Rerolls, Grappe/Bullrush/Trip or what-you-have.)
I agree, there has to be some form of advancement in a campaign setting.