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Have the PCs buy with gold their level progression at training grounds or something. Might&Magic world of Xeen used this idea.
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...experience points for treasure gained.
I understand where the XP systems of 3e and 4e are coming from and, mostly, I think they achieve their goal. However, I feel that there is a unifying feature for XP=GP that is sometime sorely missed. When the primary motivation in an adventure is Treasure(!), then you know what everyone wants and the glee of finding a dragon's hoard is more than merely "what can I buy with this stuff?" (Which, a lot of the time in AD&D, was "a follower" or "a castle" - not some bit of magical power).
Sure, not giving XP for treasure opens up a wide set of other motivations. Not all PCs need to be mercenaries. It makes the monk fit in a bit more. However, occasionally the nostalgia overtakes me and I mourn the loss of finding 1000 gold... and gaining 1000 XP!
Same here. Of course this kind of advancement did not fit every campaign but each setting could have its own variant.I miss the "You get a Keep and dudes to run it at 9th level". I like nation building and management of that style, and I haven't seen that thing in 3e or 4e at all yet.