RMAN backups, incremental, not compressed: estimate size?
In short: Not really.

An RMAN backupset (e.g. BACKUP DATABASE) will backup up any blocks in your datafiles that have never been used.  This is different from empty blocks.  There's no way that I am aware of to differentiate between blocks which are currently empty and blocks which are unused.

From: http://docs.oracle.com/cd/B28359_01/backup.111/b28270/rcmcncpt.htm#BRADV89481: 


  "RMAN always skips blocks that have never been used"


If you were to do an image copy (e.g. BACKUP AS COPY DATABASE), then this is essentially a direct copy of the datafiles.  So if the datafile making up your system tablespace was called system.dbf, RMAN essentially copies this whole file.  This means your backup will be the same size as your datafiles.