Help with abstract base types

I'd like to create a number of base classes as abstract, that would then 
be used to build elements.

The reason for this is that some restrictions are still being debated, 
etc., so I don't have exactly what may be allowed for a particular type.

Eg., I'd like to create a "stringType". I don't want anyone to be able 
to use stringType directly and the restrictions on stringType may morph 
before being locked down.

So, at the simplest, how can I use xs:string to create my stringType, 
but have my stringType be a complexType with abstract flagged true?

I'm missing something in the brain pan, because none of the permutations 
I've tried have worked.

Any help would be appreciated.

-David