currently, there isn't much point to going for the harder maps on ball race and minigolf. you get the same payout, but with more difficulty. I guess there seem to be more melons in the harder ball race maps, however if you're playing alone that doesn't matter as melons don't count in singleplayer. there should be a unit multiplier depending on the map difficulty you're playing on, but that's just my suggestion.
you might say, "but won't that mean nobody will play on the easier maps?" well, i personally think that this change will make new players play on the easier maps, to get better at the games, while the experienced players will play on the harder maps, which is in my eyes how it should be. currently, nobody in their right fucking mind will play maps like prism, unless they're masochists.
the main problem i personally see with this is that ball race already pays out way too many units, but that's not really a problem with this idea, it's just a problem with ball race's payouts.