Let me mention some things to complement your and TKR's lists.
First, the adjectives iuvenis and senex have the irregular comparatives iunior and senior.
These comparatives are rarely (if ever) used in neuter.
Neither adjective has a superlative.
For senex, the superlative can be replaced by that of vetustus (vetustissimus).
For vetus, comparatives and superlatives are often taken from vetustus.
I do not know how the missing superlative of iuvenis is typically replaced.
Some adjectives ending in -dicus, -ficus and -volus form comparative an superlative from -dicent-, -ficent- and -volent-.
- maledicus, maledicentior, maledicentissimus
- magnificus, magnificentior, magnificentissimus
- benevolus, benevolentior, benevolentissimus
For adjectives ending in -eus, -ius or -uus (not -quus) use magis or maxime to form comparative and superlative.
For example, magis/maxime idoneus instead of idoneior/idoneissimus.
There are adjectives coming from prepositions that only have comparative and superlative but no positive.
These are prior/primus, posterior/postremus, superior/summus/supremus, inferior/infimus/imus, exterior/extremus, interior/intimus, citerior/citimus, ulterior/ultimus and propior/proximus.
For adjectives ending in -er, the superlative is formed with -rimus.
- pulcher, pulchrior, pulcherrimus
- liber, liberior, liberrimus
- acer, acrior, acerrimus
The superlative veterrimus also has this ending.
Similarly, these six adjectives ending in -ilis form the superlative with -limus: difficilis, dissimilis, facilis, gracilis, humilis, similis.
For example, dissimilis, dissimilior, dissimillimus.