I killed the fighter, barbarian, cleric, monk and wizard to keep the bard, ranger, druid, paladin, rogue, sorcerer and warlock.
breakdown of how the remaining classes can support removed classes through subclass specialisation
bard => wizard, cleric
ranger => fighter, monk
druid => fighter, barbarian, cleric
paladin => fighter, barbarian, cleric
rogue => monk
sorcerer => barbarian, cleric, monk, wizard
warlock => wizard
the first two on the list are obvious imo as they're already halfway there anyways, the bard gets subclasses with expanded spell lists of arcane/divine spells and a few more supporting abilities like bonus spells for school specialisation for the wizard or a life cleric build and one that specialises in radiant damage, while the ranger gets subclasses that increase their hit-die, an extra attack, fighting styles and tanking capabilities and their skirmishing and disabling capabilities respectively.
the fighter and barbarian-druid subclasses get martial weapon proficiency, the fighter one gets additional attacks and special bonuses to wildshape forms, the barbarian ones have unarmoured defence and can burn wildshape uses to start rage as well as gaining the totem barbarian's spirit customisation, again for a cleric build you basically get the life cleric pack.
paladin is already a pretty decent fighter but the subclass gets an aura that helps draw aggro and a few extra fighting styles, similar to the druid the paladin's barbarian subclass can use their channel divinity's to start raging and gets a 'life cleric subclass' but also an extra potent anti-undead/demons/ect subclass.
the rogue-monk gets to use their sneak attack die as currency for various maneuvres, either you can blow them all in one big sneak attack or can 'spend' them individually, 2d6 for each extra basic attack you make on your turn, 1d6 to add 10ft to your movement, 1d6 to add stunning strike to your attack, 1d6 to not provoke opportunity attacks...
sorcerer subclasses for the wizard and the cleric function similar to the bard's ones, but specifically they have reduced metamagic costs for specialised spell schools and healing magic respectively, the barbarian subclass gets unarmoured defence, a hit-die bump and can induce rage with sorcery points, they get an expanded spell list filled with 'weapon spells' like the bladetrips, zephyr strike, the smite spells and such that can be used on their special natural weapons that scale damage as they level, and have a unique ability to cushion incoming damage by burning spell slots almost like an inverse-smite, and similar to the rogue-monk sorcerer can expend sorcery points to fuel multiple attacks and their other typical monk abilities.
the subclasses for the wizard specialisation for the warlock offer an expanded spell lists for specialised spell schools as well as mystic arcanum for those spells starting at lower levels, it gets ritual casting as standard and a handful of eldritch invocations for eldritch blast that offer damage type versatility and modify the area it effects a limited number of times per rest.