Maybe each beatmap after the first gets a (smallish) bonus multiplier to its rating, perhaps one whose value depends on a) the ratings of the beatmaps so far and b) how many beatmaps have been ranked until then? Mediocre ratings will give a tiny boost, while good ratings will give a larger boost. Likewise, having many ranked beatmaps will give larger boosts compared to only having one other ranked beatmap.
...You know, that reminds me a little of how combos in Ouendan, EBA and Osu! work, right? Consistent good playing/beatmapping makes the combo go up, which makes each subsequent beat(map) in the chain worth more. The only differences is that a "300" (a high rating, so to speak) makes the "combo" rise more than a "100" or a "50", and that "Misses" (Ranked beatmaps with, God forbid, rather low ratings) will still raise the combo, but only a tiny little bit. Of course, in the beatmap rankings case, this bonus needs to be really, really, really toned down compared to when playing, as this could go out of control if the formula is unbalanced. Ideally, I think that the formula should still give more weight to the actual ratings for each individual map.
...You know, that reminds me a little of how combos in Ouendan, EBA and Osu! work, right? Consistent good playing/beatmapping makes the combo go up, which makes each subsequent beat(map) in the chain worth more. The only differences is that a "300" (a high rating, so to speak) makes the "combo" rise more than a "100" or a "50", and that "Misses" (Ranked beatmaps with, God forbid, rather low ratings) will still raise the combo, but only a tiny little bit. Of course, in the beatmap rankings case, this bonus needs to be really, really, really toned down compared to when playing, as this could go out of control if the formula is unbalanced. Ideally, I think that the formula should still give more weight to the actual ratings for each individual map.