There's a lot we don't yet know about 5e mechanics, probably because the designers themselves have not yet figured out what these mechanics are going to look like. So this post is being made based on a couple of assumptions (that may easily prove to be wrong): that the themes mentioned in the recent blog post are going to be builds; that each class is going to have a relatively small number of mechanically distinct themes; that each theme has feats associated with it that provide specific abilities; and that a player can customize their character by taking whatever feats they want from other themes in their class as well as choosing to take levels in other classes.
With those assumptions in mind, I'm going to lay out what I would consider the most important classes to include in 5e and what I would see as the main themes they should have, and then provide desciptions of what I see them looking like. This is purely a thought-experiment on my part, though I would be interested to know how my conception of which classes are key or what their role is or what their flavour should be differs from other people's.
Note: In an effort to avoid class glut, I will re-classify some classes as themes. The Sword Mage I'll leave out entirely, since I feel it is probably better achieved through multiclassing. I’ve also left out the Artificer; they would make a good wizard theme in an Eberron setting, though. The Essentials assassin I classified as a rogue; the pre-Essentials assassin I classified as a sorcerer. I didn't put in a psionic class, which would have a psion, battlemind and ardent theme, because I got tired and have less interest in psionics, but it should be in there.
As a warrior focussed on mastering combat techniques, the fighter aquires various abilities that can assist them in front-line combat.
A medium armoured fighter who favours two-handed weapons, the slayer's feats provide abilities related to dealing more damage with attacks or to striking more foes
A heavily armoured fighter who favours a sword and shield, the knight’s feats provide abilities that make it “sticky”, punishing enemies for ignoring them and helping them stay alive
A medium armoured fighter who favours weapons with reach, the warlord’s feats provide them with abilities that assist their allies and take advantage of the battlefield
An unarmoured fighter who favours their fists and improvised weapons, the monk’s feats provide them abilities to help them rapidly move across the battlefield inflict debilitating conditions on their foes
A lightly armoured fighter who throws themselves into battle with a weapon in each hand, a berserkers feats provide them with abilities that trigger upon being hit and also that allow them to fly into rampage
Lightly armed and armoured, the rogue is nevertheless the ultimate specialist in dispatching single enemies once they are isolated, and their abilities reflect this.
A stealth specialist who fights short swords, the lurker feats help them stay hidden or slide behind foes to inflict massive damage
An acrobatic fighter who prefers light weapons like daggers than can be used for both throwing and stabbing, the trickster’s feats help them dodge their way across the battlefield and attack with thrown weapons
A daring swashbuckler who typically fights with a rapier, the bard feats allow taunts their opponents into leaving an opening as well as encouraging their allies to press the advantage
A patient killer who prefers to wait for the right moment and attacks from a distance or with unusual weapons, the assassin’s feats allow them to delivered carefully prepared attacks to inflict high damage or to poison their weapons to weaken their enemies
Rangers are solitary warriors who focus not on developing techniques and stratagems like the fighter or through stealth clever tricks like the rogue, but by honing their animal instincts, which they use to hunt the enemies of civilised nations out on their home turf. They are lightly armoured and skilled with both the bow and two-weapon fighting. All rangers gain feats which focus on moving quickly and taking advantage of the terrain, as well as an additional area.
This ranger ambushes groups of enemies and attacks them head-on, and so their feats provide them with abilities from attacking and defending against multiple enemies using two-weapon fighting
This ranger is a sniper, taking down enemies one at a time from a distance, and so their feats provide them with special attacks using their bow
This ranger fights with an animal companion by their side to even the odds, and so their feats provide them with attacks that work in tandem with this ally
The pre-eminent divine spellcaster, the cleric keeps the party alive to continue the fight while also offering their own combat skills. All clerics gain feats that provide them with healing magic, as well as feats suited to their own specialisations.
A medium armored cleric who fights with a mace in the front lines, the chaplain’s feats provide prayers to them strike down their foes directly
An unarmoured cleric who provides support from the rear, the priest’s feats provide prayers to strengthen their allies and weaken their foes
An unarmoured cleric who specialises in offensive magic, the invoker’s feats provide prayers to summon angels and call down cataclysmic effects
A divine warrior, the paladin is not sworn to a god the way a cleric is but has their own personal oath that they must fulfill; it is this oath that provides their attacks with divine strength and turns them into unstoppable forces.
The Defender: This heavily-armoured paladin has sworn an oath to defend others, and their feats give them abilities that allow them to protect their allies and punish those who harm them
The Avenger: This lightly-armoured paladin has sworn an oath of vengeance, and their feats allow them to bypass obstacles between them and their targets and then inflict heavy damage when they reach them
The most versatile of the spellcasters, the wizard shuns weapons and armor but supports the party with a wide range of magic
A destructive-magic specialist, the mage’s feats provide them with spells to harm a large number of foes as well as spells to protect themselves from harm
A master of misdirection, the illusionist’s feats allow them to conceal themselves and their allies, as well as to inflict damaging nightmares upon their opponents
A wizard specialising in calling things forth, the summoner’s feats allow them to call in and control powerful creatures, as well as magical effects such as crushing hands and floating swords
Unlike the wizard, who trains to master the arcane arts, the sorcerer is someone cursed by a power they struggle to control; they lack the versatility of the wizard, but make up for this in raw destructive power. They tend to be lightly armoured and fight with simple weapons.
A sorcerer in whose veins runs the blood of dragons, the dragon-touched’s feats provide them with destructive elemental magic and the ability to assume partial draconic transformations
A sorcerer who had the misfortune to come across a piece of raw chaos, the chaos-touched’s feats provide highly erratic offensive spells, as well as equally unpredictable abilities they can use on themselves
A sorcerer who was mortally wounded and then rejected by death, the shadow-touched’s feats allow them manipulate shadows against their foes, as well as to turn themselves into shadows
The warlocks gets their powers not from studying the arcane arts or by being cursed with unusual power, but through a short-cut: making a pact with some otherworldly being. In addition to magic, they are medium armoured and fight with weapons granted by their pact-holders.
As someone who has been granted power by devils, the infernal warlock’s feats grant them curses that deal lingering damage, as well as devilish abilities such as increased strength and elemental resistance
Those who have sworn pacts with the lords of the fey-wild gain feats that grant them curses to bewilder their foes, as well as fey-like abilities such as teleportation or charming gaze
Those who have sworn pacts with unnatural celestial bodies gain feats that grant them curses that make foes insane, and abilities that can transform their bodies in horrible and unexpected ways
The druid draws their magic not from the gods or from arcane rituals but from the world around them, spells which can both aid their allies and hinder their foes. They are unarmoured and lightly armed.
The sentinel watches the world from a distance and gently guides the forces of nature rather than becoming involved themselves, and so their feats allow them to call animals to their aid or to launch weather based spells to damage their foes and help their allies
The guardian prefers to throw themselves directly into the affairs of the world to protect nature, and so their feats allow them to transform into animals and to alter the battlefield around them
Shamans call forth spirits that assist their allies in better, and so their feats allow them to cast healing and protective spells through their spirits, as well as to manifest their spirits to assist them in combat