you can do that with merging with an empty espace already.

keeping the data is similar to this idea: https://www.outsystems.com/ideas/72/

Joost, I think that what Gonçalo is requesting is not the same that you are suggesting.

If you are working on an eSpace that turns out to become very big and you want to separate the data model from the web interface, merging into a blank eSpace is not the ideal option, since the new eSpace (with the web interface) will not reference the old eSpace entities; entities will be copied to the new eSpace as well. You would have to reference the old eSpace entities, Find & Replace usages, and so on...

The splitting option seems to be very interesting!

At NextStep 2010 I visited a presentation about refactoring eSpaces. You can find the presentation here. 

It was quite impressive how fast you can split eSpaces and how the platform selfheals. Although  you have to know how to do it. You have to do the steps by hand and you must know how to do it. There is no automatic mechanism that does the refactoring for you.

So there are refactoring possibilities in 5.0 but hopefully this will be enhanced in 5.1.