These are bascially listed in order from “smallest and easiest” to “biggest and hardest.”
Step One: Eliminate the lost spellcasting level at 3rd
There’s literally no reason for it. At the time, it may have been believed that there were advantages to spontaneous casting that demanded it as a balancing factor, or, as rumored, Monte Cook may have just hated sorcerers (see here for a search for the real reason), but experience and hindsight have demonstrated it to not be the case.
Step Two: Rapid Metamagic
One of the natural and fitting advantages that spontaneous spellcasters should have is the ability to apply metamagic on the fly, but the increased casting time for this ruins that. So eliminate that.
Step Three: Consider More Spells per Day
This is the advantage that spontaneous casters have, but the difference isn’t all that large: usually about 1 spell per day per spell level. It could be bigger, to make it worth a little more. That said, spells per day is rarely a meaningful limitation on spellcasters past level 7 or so (and even that might be generous).
Step Four: Consider More Spells Known
It definitely reduces a major stumbling block for them, and considering that prepared spellcasters have spells known of “all the spells,” they aren’t likely to be threatened by it. For example, sorcerers know spells in a distribution very similar to the wizards spells per day, which means that the sorcerer has to “prepare” the same number of spells as the wizard, he just has to prepare for the rest of his life while the wizard only has to prepare for the next day.
Bonus: Sorcerer specifically
The sorcerer really deserves a better chassis and class features. 2 + Int skill points, on a Cha-based class? That’s just mean. For that matter, a Cha-based class, with only Bluff as a Cha-based class-skill? Why? Give them Intimidate, at the least. Some more knowledges would not be amiss. And some class features, even if it’s just Eschew Materials at 1st and bonus feats as the wizard gets. Some of Pathfinder’s bloodlines are ridiculous, but some of them are fair enough.
From the Player's Basic Rules, page 71 (or PHB p. 191):
In contrast, you can move through a hostile creature's space only if the creature is two sizes larger or two sizes smaller than you.
An ogre is a large creature and elves are medium. So an ogre can't move through an elf's space. In your previous example, if we have a 10-foot wide passageway with two elves standing side by side, the ogre can't get through without killing one of the elves or forcing them to move.
The rules on squeezing into a smaller space are for an ogre trying to move along a 5 foot corridor. The rules on creature size that you've quoted back this up—the ogre isn't actually 10 feet wide, that's just the space he controls. So he can move through a 5 foot wide gap, but it's cramped and he can't move freely.
Now, you might be thinking that it's a bit unfair on the ogre if the 2 elves can form an impenetrable barrier against it. As you've said, he'd rather shove them aside than squeeze between them. And he can do just that! From the Player's Basic Rules, page 74:
Using the Attack action, you can make a special melee attack to shove a creature, either to knock it prone or push it away from you. If you're able to make multiple attacks with the Attack action, this attack replaces one of them.
The target of your shove must be no more than one size larger than you, and it must be within your reach. You must make a Strength(Athletics) check contested by the target's Strength(Athletics) or Dexterity(Acrobatics) check (the target chooses the ability to use). If you win the contest, you either knock the target prone or push it 5 feet away from you.
So you're right, shoving is the ogre's answer here.
Now, with your new example, (2 elves in a 15 foot corridor with a 5 foot gape between them), things are different. The ogre does indeed have to squeeze between them. Fortunately for him, it's not going to matter all that much.
Why? Because he's almost guaranteed to be moving on his own turn. Unless there are more enemies than just the elves, he's not going to provoke any opportunity attacks while squeezing, and he can attack before or after he squeezes. So the only squeezing penalty that is actually going to apply to him in this situation is the double cost for movement.
Note that if he stopped between the elves, all these penalties would apply to him. That makes sense though—standing between two enemies with not enough space to move around in would make it difficult to dodge attacks or attack effectively.
Best Answer
Short Answer:
Basic Evocation Wizards are now the equivalent of Tier 3 at lower levels.
Basic Champion Fighters are now the equivalent of Tier 3 at lower levels.
The Usefulness of Tiers will have to change for 5e, as tiers 1, 5, and 6 at this time don't exist, and likely won't exist as the overall power of 5e is much flatter. We can compare classes by looking at their average DPAD (damage per adventuring day).
For overall power however, it's going to really depend on level.
Basically at level 11, the Fighter is doing more damage than the evocation wizard who is focused solely on damage. However, by level 17, both the cleric and the Wizard have enough spell slots, to out perform the Fighter and Rogue. In addition, level 9 spells are by their nature "game changers". It should be noted that at level 20, with the third extra attack, the fighter shoots up in power as well, though not enough to out perform spells such as meteor storm, the nuclear weapon of D&D. One caveate to the current graph is that not all levels are shown, only the level when the "adventure tier" changes. For example, not shown on the graph is level 10, where the fighter still only has one extra attack, the wizard and cleric are doing about the same amount as the fighter.
So while it is still the case at the highest levels that magic is more powerful than the mundane, up until the epic tier, this problem does not really present itself.
This obviously doesn't take into account the non-damaging effects of spells, such as haste, or invisibility, or flying, etc. However, with the new concentration mechanic, these spells are situational and can not be easily piled on top of each other as they could in 3rd edition, to create a quadratic power curve.
Relevant data for the chart:
The above chart is based on the various classes an unlimited number of enemies with the following defenses, and no assumption is made about the HP of the monster, so damage is "unbounded" in the terms of the spreadsheet:
Dex Save 2
AC 15
Con Save 2
However, If I change the defences of the monsters to something less reasonable in the game, such as giving a Dex and Con save of 5 and an AC of 10, we see the fighter keeping up with the wizard and again surpassing it at level 20.
You can compare these 5e graphs, to this 3.5 graph and notice how the fighter is at the bottom of the graph and stays very linear, while the wizard has a slight a quadradic curve in power gains.
Old Long Answer about Tiers:
First, the assertion that a Wizard might be tier 1:
It's pretty clear that currently, the Evocation Wizard can not do absolutely everything. Because of the concentration mechanic, the Wizard can really only do one cool thing at a time. You can't fly and be invisible at the same time without another magic item or wizard casting those spells on you, for example. Secondly, while some spells can duplicate other class abilities, such as "knock", those spells are only situationally better, and come at a high resource spot (limited spell slots) that other classes lack. So the Wizard is no longer a tier 1 class.
Instead, I would say that Tier 3 fits the Evocation Wizard.
The bolded part is why I believe a Wizard is Tier 3. A wizard can, if picked the right spells, do anything another class can do (with some exceptions), but it can't do those things as well as the class that specializes in that thing. An invisible wizard is hidden, but can't sneak. A wizard with damage spells, can't kill, a single high hitpoint target as well as a fighter can. A Rogue will have better skill abilities when it specializes in those skills than a wizard can ever get.
Next the assertion that a fighter is tier 5.
Tier 5 is defined by:
No class is good at only one thing. Because of the Background system, all classes and characters are able to focus on skills that are outside of their class. So while you can make a fighter that is only good at fighting, and can't do anything during exploration or social interactions, it's also more likely to build a fighter that is good at fighting and leading people. Or good at fighting and using certain tools. Because of bounded Accuracy, all characters are decent at whatever skill they are proficient in. The champion fighter is best built for two handed heavy weapons, but they can also be a good archer, or duelist. Fighters excel in combat and at higher levels, can move across the battle field, hitting multiple creatures in one turn in the same way that a wizard might target more than one creature with a spell. They are also able to survive situations that others would not without a cleric. So I don't think they can be called Tier 5.
Champion fighters are however tier 3, because they are
Fighters do combat really well, they survive well and can kill well. However sometimes a situation will be called where there is no combat. Fighters can excel at jumping across chasms, or climbing/swimming out of tough spots more so than wizards and clerics. With Background again, they can also call upon their traits. In the starter set for example, the Fighter is a folk hero, who is able to rally others to their cause.
However, the tier system as a whole is a bad way to judge D&D 5e classes. The power level of the entire system has been made "more flat", and there will need to be better definitions of how to compare classes.