Perhaps offer a config file to fly to custom planets a few mods have custom worlds and perhaps flying there instead of using a portal would be nice. The config file would be there to prevent people from flying to the nether or the end. But could fly to worlds Such as this mod which is in an early alpha stage aswell: http://www.minecraftforum.net/topic/1553355-146147-prometheus-mod/page__st__100
Details:
This is how I would imagine it working. The mod has a custom Menu page for all custom worlds (worlds not added by your mod) which can be accessed by pressing a button at the default outer space menu that you reach when you fly into the atmosphere. In the custom menu it could either be just a list of worlds as buttons to press or (Insert more ideas here) which would then teleport you to a Y distance of (usually max sky limit) in which you would parachute down like you would on the moon. Plenty of mods add custom world and having a more exciting way of getting there would be better then boring old portals you walk in and out of.
Details:
This is how I would imagine it working. The mod has a custom Menu page for all custom worlds (worlds not added by your mod) which can be accessed by pressing a button at the default outer space menu that you reach when you fly into the atmosphere. In the custom menu it could either be just a list of worlds as buttons to press or (Insert more ideas here) which would then teleport you to a Y distance of (usually max sky limit) in which you would parachute down like you would on the moon. Plenty of mods add custom world and having a more exciting way of getting there would be better then boring old portals you walk in and out of.