I know it's possible, but I can't save presets outside that world and it's a pain to cheat in all the dimlets - you can't have NEI/the creative menu AND the Dimension Enscriber open at the same time (or at least, NEI is small on my monitor when that's open). The ideal world type for this would have the search bar open at the same time as the "enscriber" and always able to be typed in; you can't click out of it.So you want to replace your Overworld with an RFTools dimension... interesting.
It's possible to fake that by setting your spawn point in an RFTools dimension, which is already possible (although it's not so reliable in SMP... either that or my ferver admin friend derped). Gideonseymour pulled it off in Pathfinder, somehow... I know that there is a "Special Spawn Dimlet" that allows you to respawn in an RFTools dimension as long as it's not low on power. However, I don't know what happens if you die in a different dimension, use the End portal, or join a server with one of these dimensions for the first time.
Hmm, unrelated idea: Some way to create a dimension closer to the style of vanilla Superflat dimensions. I'm thinking this could take the form of a Terrain dimlet that takes an arbitrary number of Material and possibly Digit dimlet modifiers, so [Digit 1] [Material Bedrock] [Digit 3] [Material Dirt] [Digit 1] [Material Grass] [Terrain Superflat] would produce the classic vanilla bedrock-dirt-dirt-dirt-grass Superflat world that's constantly overrun by slimes.
Probably not very useful, and may not even be possible with the existing infrastructure for terrain dimlets, but it's an idea.