Are there certain modern stamp oddities (recalls, errors, color shifts, etc) that will always be desirable?
For US Issues
Oddities of rarity such as the non-inverted bi-plane. No mistake but so few printed. Same with the perf 12 JPJ 1784b can be had now for under
$3500.00, had sold higher but will remain in demand. Should the unique plate block be broken into singles, that would alter the supply side for singles.
Recalls, well that one's price has drifted down.
Errors, the so called major errors (color missing, wrong color, inverts, perforation missing) will stay in demand but it may take years to see the quantity available be truly determined. But remember some true errors still have little value to begin with such as things missing you can regularly see or carries little interest such as missing gum errors on gummed issues.
The above two can be value driven by the inclusion or exclusion of the variety in a standard US stamp album.
Color Shifts, no. Once found, they tend to stay at that price level forever and is subject to later finds for years.
Etc., too vague to answer.
Interest in Errors, freaks and oddities does vary over time, thus the price will fluctuate based upon the interest and related demand.