Purple book DDCD equivalent and HD-Burn could allow 20 GB with 2.5× density on one DVD DL. BD-XL could grow to 256 GB assuming 1/2 density. HVD could hit 16 TB.
An 16 cm or 20 cm HVD (Sony ArchiveDisk size) could reach up to estimatedly 50 TB.
Error correction would be weaker of course. Physical degradation can destroy data much easier. So the opposite (LD-Burn for lower density) could allow more data safety.
I also doubt, how well it can be implemented onto DVD-RAM.