MechaTarrasque
Hero
I would probably power up fighting styles, make a lot more of them, but have them take concentration. Fighter's big gimmick would be Martial Secrets (they have access to all fighting styles) and they get a few more than other classes. Versatility becomes the fighter's gig.
I always figured warlocks should be about things being easy for the PC. Transformations seem like the easiest thing: use your cha modifier for your strength modifier if you have a fiend patron and change into a fiend shape, cha modifier for dex modifier if you have a fey patron and change into fey shape, cha modifier for con modifier (plus temp hit points) for undead patron/undead form, cha modifier for int modifier (plus prof in one int skill and telepathy) for goo patrons/aberration form, and cha for wisdom (plus once a day commune without using a spell slot) for celestial patrons/celestial form. Invocations would be more about boosting your form. The downside (since I figure patrons want to be paid) is that bad stuff happens if you use the abilities too often during the day when you aren't fighting (and dedicated the pain you cause to your patron): grow tentacles for the goo patron, bad temper for the fiend/undead patrons, easily distractible by music or pretty nature scenes for the fey patron, urge to give away your gold to widows and orphans for the celestial patron.
Barbarians' big gimmick has two parts: the rage fighting style doesn't require concentration for them, and they can cast one specific concentration spell when they rage (fear for berserker). They have advantage on concentration checks for the specific spell, and they can keep it going if the rage fighting style ends before the spell does (no advantage on concentration without rage). Alternatively, they can use a second fighting style instead of the spell (ditto advantage on the concentration checks for the spell).
Monks would be full casters who can deliver touch spells with an unarmed strike (which they can automatically attempt with any spell with range touch). The basic monk spell list would be touch spells and transformation spells with range self. Subclasses would get other spells, so the elemental monk can toss fireballs and sword monks can cast elemental weapon. Monks would have fighting styles.
I will think on the other classes.
I always figured warlocks should be about things being easy for the PC. Transformations seem like the easiest thing: use your cha modifier for your strength modifier if you have a fiend patron and change into a fiend shape, cha modifier for dex modifier if you have a fey patron and change into fey shape, cha modifier for con modifier (plus temp hit points) for undead patron/undead form, cha modifier for int modifier (plus prof in one int skill and telepathy) for goo patrons/aberration form, and cha for wisdom (plus once a day commune without using a spell slot) for celestial patrons/celestial form. Invocations would be more about boosting your form. The downside (since I figure patrons want to be paid) is that bad stuff happens if you use the abilities too often during the day when you aren't fighting (and dedicated the pain you cause to your patron): grow tentacles for the goo patron, bad temper for the fiend/undead patrons, easily distractible by music or pretty nature scenes for the fey patron, urge to give away your gold to widows and orphans for the celestial patron.
Barbarians' big gimmick has two parts: the rage fighting style doesn't require concentration for them, and they can cast one specific concentration spell when they rage (fear for berserker). They have advantage on concentration checks for the specific spell, and they can keep it going if the rage fighting style ends before the spell does (no advantage on concentration without rage). Alternatively, they can use a second fighting style instead of the spell (ditto advantage on the concentration checks for the spell).
Monks would be full casters who can deliver touch spells with an unarmed strike (which they can automatically attempt with any spell with range touch). The basic monk spell list would be touch spells and transformation spells with range self. Subclasses would get other spells, so the elemental monk can toss fireballs and sword monks can cast elemental weapon. Monks would have fighting styles.
I will think on the other classes.