Hello. I would like to preface my post by saying that I am a Senior Developer of the Pixelmon mod, and that we have recently transitioned our Pokemon spawning from using just biomes to utilizing the BiomeDictionary system, to allow for greater compatibility with other mods. While the dictionary is a good system for cross-compatibility of mods with biome generation code, we have found that it could use a few tweaks for maximum compatibility.
The major issue we have encountered is that it isn't possible to differentiate River and Ocean style biomes from tags alone (both are [Water] or [Water, Frozen]). I would like to propose possibly differentiating the WATER tag into RIVER and OCEAN (possibly retaining the WATER tag as referring to either for compatibility, but it may be necessary for mods to adapt to the new tag split). This would allow for the following vanilla initilization:
BiomeDictionary.registerBiomeType(river, RIVER);
BiomeDictionary.registerBiomeType(ocean, OCEAN);
BiomeDictionary.registerBiomeType(frozenRiver, RIVER, FROZEN);
BiomeDictionary.registerBiomeType(frozenOcean, OCEAN, FROZEN);
A secondary improvement on the dictionary is the way vanilla biomes are classified and initialized in Forge. In particulare the icePlains and iceMountains (both just [Frozen]), and the hills sub-biomes (which lack the HILLS secondary tag). I would propose changing the initialization to the following:
BiomeDictionary.registerBiomeType(icePlains, FROZEN, PLAINS);
BiomeDictionary.registerBiomeType(iceMountains, FROZEN, MOUNTAINS);
BiomeDictionary.registerBiomeType(desertHills, DESERT, HILLS);
BiomeDictionary.registerBiomeType(jungleHills, JUNGLE, HILLS);
BiomeDictionary.registerBiomeType(forestHills, FOREST, HILLS);
This initialization still allows for each biome to be identified correctly by mods that use isBiomeOfType() calls, and provides additional specificity for mods that want to differentiate the hills portions and the two ice biomes.
I realize that some of this may end up changing slightly with the new 1.7 and beyond biomes, but hope that this would improve the BiomeDictionary system in its current state.