Heavy is in a good place he just has a boring playstyle. He needs new unlocks more than anything.
Spy needs buffs to his stock playstyle and bugfixes. Pyro matchup is too one sided. Sniper should be easier for him to kill. I don't think buffing the revolver slightly would hurt either.
Pyro needs to be skill indexed. Buff the jetpack so pyro has some air control and it isn't useless in combat. Remove the range limit on the DF but make the hitbox a lot smaller and give it an arc.
Engineer could be made into more of a generalist. Speed pads and mini dispensers would go a long way to achieving this.
Sniper needs to be weaker at close range and missing should be punished more heavily considered spy is pretty much guaranteed to die if he makes a mistake. Reverse falloff of some kind and slower firing rate would be a good start IMO.

They left some things out of the patch notes. Flamethrower damage was doubled across the board.
https://wiki.teamfortress.com/wiki/Flame_Thrower
Previous value was ~6.82 if I remember correctly. The demonstration video still shows the old damage.

maybe you shouldn't have removed team scrambling idiot
valve logic

Are the other gamemodes out of the question assuming their flaws could be fixed?

Have to agree on the ambassador changes, just make it mini crit instead and everybody will be happy.

A promod would be neat but I can see a few problems.
1. Getting the client to be standalone will be difficult.
2. Some of the worst bugs in TF2 are engine related and can't be easily fixed.
3. What should be taken out and what should be left in?
I think what TF2 really needs at this point is a spiritual successor.
Maybe when source 2 finally comes out.

Does this work in Valve MM?

What is with this mentality that "the competitive community needs to contribute more"?
Valve has been halfassing this game for years and they've milked mountains of cash out of it. Valve needs to step up to the fucking plate for once. They're not some tiny indie company anymore, they have a monopoly on PC gaming and are worth billions of dollars there's no excuse for this. Have you forgotten this is the same company that just last week reintroduced sprays into csgo with a price tag and a fixed number of uses, that tried to force paid mods on us, that runs a massive e-gambling scene and takes a cut of every profit, that had months of beta testing only to not fix anything, that repeatedly delays every update, (Remember how matchmaking was "supposed" to arrive in august 2014? Remember those comics?) that left gamebreaking bugs and balance changes in the game for MONTHS at a time. Have you all forgotten about the old loch n load and short circuit? What about the sandman that used to be able to stun ubers? Hell as for that matter why does the reserve shooter and dds still exist in their current state? It should be blatantly obvious to anyone who has spent >10 hours in TF2 just how utterly unfair these weapons are. What more is it going to take for to people wake up? Gabe hasn't been the "SAVIOR OF PC GAMING" for a long time now. The Valve that made TF2 9 years ago is not the same as the Valve today. You claim that we should be patient and that TF2 is too big to change overnight, why not work on a new game then? TF3 is well overdue by now but of course we all know that's never going to happen because valve thinks they're above making video games now and would rather work on their VR gimmicks and coming up with new ways to psychologically manipulate kids into buying overpriced cosmetics with their moms credit cards. I will give you that it is a lot of work and there's many things that need to be fixed but whose fault is that? It's not like we realized last week the game is a bloated horrendous mess, it's been a slow decline year after year. Valve has 0 QA when it comes to TF2. Face it Valve only works when money is on the line. As much as I hate to say it people like delfy and sin were right. Valve won't get off their asses until the shitstorm is on their doorstep. You shouldn't be saying "how can we appease you Valve?" you should be saying "if you aren't going to communicate with us then fuck off". Valve has no right to dictate how the game should be played competitively when they've had no desire to contribute since day 1. Not that I'm saying we shouldn't make compromises at times, the format is far from perfect, but this late in the game it's up to Valve to prove that they're up for the task. What they have been doing for the last few years isn't going to cut it anymore. Just removing the config whitelist on comp and communicating with us would go a long way. I love this game to death but at the same time it's infuriating to realize just how much Valve has squandered TF2's potential.

Sorry for the block of text. Just wanted to get that out there.