Increase drop rates and give more money at the end of explorable dungeon. With the increased difficulty, it is appropriate to have more rewards compared to story mode dungeon. Those are the only problems I have. Diminishing reward is okay; there are a lot of paths to do anyway.
Not everyone runs dungeons to get the armor set; it is expensive and gives no actual benefit. Some prefer to do it for fun. It does not have a good mechanics so challenging factor is non-existent, therefore the only fun factor left is rewards. It does not reward players well either, so there is no fun factor left in dungeons anymore. This is however, is my opinion.