My question, and I'm not claiming there is or isn't a "right" answer is... would these different "paragon paths" have a mechanical effect?? Or are they just window dressing for a single classes mechanics?
Both? Some will be window dressing for archetypal fighters (who are big strong and tough) and others like the fey will have mechanical effects. And if you're playing 4e a third category will be expressed through the selection of powers rather than themself having a direct effect.
If they want to keep the 4E audience, and this is the route chosen, then they'd better put the "mythic fighter" class and the "regular fighter" class in game from the get go.
Honestly I don't think that 4e players all require mythic fighters
as long as fighters are (a) fun and (b) able to keep up with wizards. The problem is that big, solid, and hitty
needs to be mythic in order to keep up with wizards.
If the fighter is a spam class (2e, 3e) or worse yet barely more competent at combat than an unbuffed cleric who is spending his healing on others and contributes little more than a warm body to the other two pillars (1e pre-Unearthed Arcana) then the 4e players will revolt.
If the fighter turns out to be near-mundane in the way
Indiana Jones is near-mundane then I don't think we'll have a problem from the 4e side. Although I think the old school players will complain - playing Indy would be far too complex for a simple fighter.
If you want simple you either need utterly lethal (seriously, start with two attacks/round) or a lot larger than life.
Simple, mundane, effective. Pick two. And 4e players are not compromising on effective.
The reason it's pick two is that to take things on above your weight class (as you need to if you are mundane) you need to work
hard. And preferably smart.
Poison? Or an antidote; to the insidious poison of power creep in the game and quasi-magical wahoo for non-magical classes?
If you want to deal with power creep, take the wizard and feed it into a woodchipper. Then burn the parts, and scatter the ashes. The insidious poison of power creep has been benefitting wizards ever since D&D started. Go back to the Illusionist as a class and have the Evoker as a separate class.
Hyperbolic, yes; but your definition puts me in the "poison" category (I think fighters in 5e core need to be bog simple "hit it till it's dead" characters that anyone can pick up and play in a heartbeat) and that does tee me off.
Simple. Effective. Mundane. Pick two. If the fighter is to be effective at combat he needs to either be tricky or powerful.
No one is saying that there shouldn't be a "hit it til it's dead" class. That is a purpose of the
mythic fighter. The one who can think with his muscles rather than his brains. In order to be a mundane fighter in a magical world, the fighter needs to think his way into levelling the playing field. And that takes being tricksy rather than hitting things until they die. (I'd argue this role is better served by a rogue variant).
No other class can really fill that simple entry-level role.
Nonsense. A simple blast mage
works. 4e has a simple entry-level blast mage (the Elementalist Sorceror). There's another class that can fill that role.
Also, I'd like there to be at least one class (but preferably, all of them) where I as player don't have to remember powers or look at skill lists etc. - I can just role-play the character as entertainingly as I can, roll some dice when I have to, and let the DM tell me what happens.
How does "Single target firebolt" and "Fireburst" as your two spells sound? Oh, and Affect Normal Fires. Absolutely nothing that doesn't fit your criteria.
I have to ask - and I can't quite believe I'm about to type this - how can you possibly balance those in the same game? I don't think you can, thus either one has to accept imbalance in the game or one of those classes has to go.
By making the mundane fighter tricksy. Seriously tricksy. You end up with the mythic fighter's
Secutor to the mundane fighter's
Retiarius. Or the mundane fighter's Batman.
It's probably easier to balance the mythic vs mundane fighter than the fighter vs the wizard. But in order to be both simple and effective you need sufficient brute force that a simple brute force approach works. You absolutely can not do that in a world with ogres and dragons without making the fighter more than mundane. Or level capping him. Or making him a ball of death that would make a 2e dart master wince.