What fixes would be required if WotC were to suddenly drop 5E and try to make a 4.5E that fixes all the problems of 4E without completely destroying it and starting over from scratch?
It won't happen. Not from WotC.
What fixes would you want and what features would you like to see?
I'd like to see a lot of failed ideas thrown out. To give a linked pair of examples: in a Greyhawk 4e campaign where I'm a player, one PC is a bravura warlord. He has an at-will power called Brash Assault. Here's the effect:
The target can make a melee basic attack against you as a free action and has combat advantage for the attack. If the target makes this attack, an ally of your choice within 5 squares of the target can make a basic attack against the target as a free action and has combat advantage for the attack.
That works out alright. Not my favorite power, as it relies on enemy behavior rather than PC behavior, but it will work reasonably well, as enemies seeing you in a "weak position" will usually try to take advantage of it.
There's also a Dragon Magazine feat, Harlequin Style. The relevant text:
Brash Assault (warlord): Against the target of this exploit, you gain a bonus to defenses equal to your Charisma modifier until the start of your next turn.
So you can get that free attack, at a significant penalty. Almost like an obvious trick. Unsurprisingly, the DM, "M", stopped attacking. It took two sessions before the player retrained the feat into something that wasn't taking from being a warlord. I don't know why that feat even exists.
Something similar came up in my campaign. "M's" PC died two sessions ago, and his new PC, introduced last session, was a heavily-armored fighter. One of their foes had cool armor, and I had previously asked what type of armor it should be (so the PCs could pry the prize off his corpse, not that it was easy). But none of the PCs wore heavy armor except the new PC, so no one had been able to request the armor. I gave the NPC some coolish armor, and felt a bit bad because he was a higher-level soldier, and I just gave him +1 AC.
"M" wanted armor that gives you +2 to defenses against anyone you mark. In other words, you optimize yourself out of a job! Enemies would start ignoring his marks because the only penalty to that is Combat Challenge, which enemies won't always know about (at least not until you punish them for the first time). I explained my objection and said no to that. He chose another type of armor instead, and (due to an adventure-prescribed plot event) the armor actually changed to suit his needs.
4e got too big, and there's a ton of crud gumming up the works. Too many poorly-designed, weak and even stupid (vampire!) classes, plus the same thing when it comes to feats, items, powers, etc.
If someone Pathfinderized 4e, for me it would be a way to get a far more limited Character Builder. I hate the CB. I think it takes away DM authority. Apparently I will never be able to teach my players to avoid certain sources, and instead have to audit character sheets and say "that power/feat/item/whatever" is broken. Broken being a relative term, as 4e's math does work far better than it's predecessor's, but still.
For me I'd like to see:
A way to make a class have any role. So you could have defender Wizards and Striker Paladins with just a few choices at character creation.
I wouldn't like to see that. If there needs to be a wizard defender, that should be its own class. Actually, that
is it's own class, the swordmage, and I have one in my campaign. It works pretty well. (There actually is a striker wizard of sorts, the Evoker subclass, although it's striker feature is fairly weaksauce.) There's no "striker paladin", although an avenger is sort of close. If there needs to be a striker paladin, I'd like to see a new class, not a modification on a defender. I don't want to see a heavily armored striker that can still take powers that give it Divine Sanction.
A way to speed up combat without losing the range of choices for each character.
That would require rewriting all the powers, not that I would have a huge problem with that, as long as rewrite doesn't mean "throw out everything cool". If there were no off-turn or minor action attack powers other than class abilities, I'd be happy. Does the game really need Disrupting Shot? Or a runepriest's ability to blind someone as they attack an ally?
I'd also like to see basic changes to a few things, like action points. They're supposed to be powerful, but I start thinking they're just cheese.
Good software tools to simulate all the math and rolling.
Yuck. I have other problems with the CB other than the one I mentioned above. If you have to use a CB to play a character, you haven't actually learned the game.

I had to teach a 4e player (who later became a DM) how to play without the CB, as they hadn't even learned proficiency bonuses because the CB did that automatically for him.
I'm bad at math. Another player complained about how long we were taking totaling up the dice, and he rolls attacks and damage simultaneously. It took me about three sessions to master rolling attack and damage dice simultaneously, and I say this as someone who got 52% in grade 12 math. A big deal, because my current PC is a wizard, and all but one of his attacks is multi-target. This sped up my DMing too, although alas all that rolling still overwhelms me when I'm playing a solo monster. (I will probably use average damage for all solos I use rather than rolling for damage.)