Keep things simple. Simplicity is the key to changes. But that's my opinion.
We can keep things simple by extending it from 00-15 to 00-98, with 99 reserved. However, what would these extra colours be?
Use the EGA colour table. That's 64 colours, and assign numbers 20 through 83 to those numbers. Whether they'd be sequential is another issue. Perhaps matching colours close to what they'd normally be would be best. For example, 24 is currently the colour yellow, so I would suggest one of those 64 colours to be close as possible to that colour.
