Should be closer to 1 added every 8 - 10 years.
Agree with this. Id probably say 2 every 10 years, but to keep it fresh in people's minds, I'd do 1 every 5 years. I reckon the only reason they've done 2 every 4 is so that they've got enough spaces to get JT, Smith, Slater and probably Inglis (who I violently disagree with) in.
2 every 4 years will soon see the numbers pile up and it them dilutes the standard of Immortals and also reduces the prestige of the HOF.
A bit like AFL, Immortals should only ever be a maximum of 10% of the HOF, and I'd even argue that's too many - would be more comfortable with 5%, and only move away from that in extreme circumstances, but then get back to the 5% over following years.