Most of your points are valid and should make it into the update regardless of what Reforged does. Current update is done without Reforged in mind (although I will refrain from importing many more custom assets), and later I will do a thorough visual pass with Reforged assets and improved cinematics.
1. Campaign is short: The book is short ;-) My plan is to redesign almost all maps. There will be fewer maps, but a lot bigger (like 2-4 times as large as before), chapters 2-5 are planned to have around 60 minutes of playtime and chapters 1, 6 and 7 will have around 20-30 minutes each. I plan to add many more quests. I will also add a new Interlude between Chapters 5 and 6.
2. More boss fights: I would love that. I am not good at creating them but I will look into having more.
3. Flashback with Doomhammer fighting Lothar: I try to refrain from adding flashbacks about important past events unless they were part of the book. I don't want to distract from Thrall more than necessary.
4. Prologue needs change: The prologue with Gul'dan is going to be part of another cinematic when Durotan explains himself to Doomhammer. I will remove the "Storyteller" to make room for "active" cinematics ("show, don't tell"). Medivh/The Storyteller will still appear at the start and end of the campaign. "Blackmoore finding Thrall" will make a comeback as the Prologue of this campaign, but it will be much shorter than before. Later in the campaign you will experience the full quest with Draka and Durotan.
5. Balance changes to Spears and the Ogre Blade: I will look into them. I might add more "normal" enemies to each fight. That will depend on how much fun I can make those first abilities for Thrall.
6. Arthas cameo and Taretha in prostitution: Is Arthas really in the book? I have to look it up again^^. I might add a short dialogue sentene by Taretha that "Blackmoore used her for ... other things".
7. "Hellscream" interlude as playable chapter. I have often thought about this and there are good arguments for it: You could have a series of challenges for Thrall and it's about the only map where he might fight orcs, but I would refrain from having Hellscream as the bossfight. On the other hand I really like having an interlude at that point as a breather... but it is quite a long interlude... maybe I can do both: Add some challenges and then have the interlude with Hellscream talking to Thrall.
8. No use for the Dog in "Unchained": I plan to add some fighting capabilities to the dog. You will probably rescue him a bit later, maybe outside Durnholde.
9. No point in wolves in "Lethargy of the Wolves": I see that as the first signs of Thrall's connection to the Spirits of the Wilds. The companion system will make it so you will only have one Wolf at a time.
10. Replace imprisoned grunts with peons and non-human guards with human guards. I would be bothered by the sudden change of their physique once they acquire weapons. I can see a story-reason for having only humans, but for variation I owuld keep elves and dwarves. Also, looking at the WC3 campaign the alliance of races doesn't seem to be that splintered...
11. Remove hide from Spearthrowers: Yes. Probably add some other companion upgrades.
12. Running from cold not fun: Hmm, but I like it because it puts some pressure on Thrall 
Maybe I will just make it a strong debuff and not instant death. I'll see.
13. Quests like catching pigs should be optional: I want to add more mini-games actually, but I will make all of them optional.
14. Drek'Thar boss fight: As an ally maybe.
15. Remaking elemental quests: I will make all main quests more "normal", but I would like to have some sort of flavour for each so it doesn't get boring over time. The elemental quests as you know them right now might actually become optional quests. The terrain will change accordingly, but in the current state the editor doesn't offer multiple cliff types. Current plan for the main quest line is to connect all areas and have an escort quest with Drek'Thar who is following you to each area. After completing 2 or 4 of the elemental quests he will offer to talk to you about Thrall's parents. Then you will play as Durotan and Draka. Probably split into two quests. These may or may not be optional quests.
16. WoW models for elemental lords: I'm careful with this. Often implementing custom models seems alluring but more than often I think they can look weird. We'll see what Reforged has to offer.
17. Remake Doomhammer bossfight: In my opinion with boss fights it really comes down to having many area of effect abilities for the bosses, so the player doesn't just spam his own abilities but is also forced to move around. I might look into creating a proper template for this so I can create many good boss fights more easily.
18. The Horde Awakens mechanic: As someones suggested I will probably use this mechanic for the final chapter, because it's more fitting when sotrming Durnholde. Durnholde will be redesigned accordingly. You should see this redesign in Chapter 2, too. So they won't be two separate maps. The current Horde Awakens map will be remade to include multiple internment camps that have to be infiltrated. No detailed plans yet.
19. Last chapter "Confrontation with the Past" is good: I like it but I think it doesn't offer much for the story, which is why the final fight with Blackmoore will happen right at the end of the Siege of Durnholde. The gameplay elements of "Confrontation with the Past" will be split and spread across the other maps of the campaign.
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New ability system:
- Combat abilities are no longer learned via skill points. Neutral Shops are used to choose 1 of 3 abilities for each hotkey (Q, W, E, R). Abilities can be switched at any time at an Ability Shop, but it costs gold. Main quests will unlock new abilities.
- Combat abilities are upgraded in Neutral Shops for gold. Normal abilities have 5 levels. Ultimates have 3 levels. Gold can be obtained by killing enemies. Main quests (and sometimes optional quests) will unlock new levels for abilities.
Q abilities:
- Knockback: Stuns, knocks back and deals damage to an enemy.
- Raging Strike: Thrall quickly moves to a target point damaging all enemies along his path
- Chain Lightning: as before
W abilities:
- Earthshock: Sends a shockwave that damages and slows all enemies in a line.
- Leap: Jump to a target location to damage and slow all nearby enemies
- Feral Spirit: as before
E abilities:
- Rage: Like Bloodlust, but can only be used on Thrall. Lower cooldown and no auto-cast.
- Challenge: Decreases armor and attack of an enemy unit and forces it to attack Thrall.
- ?: Probably not Far Sight
R abilities:
- Indestructible: Adds damage and hit point regeneration to Thrall
- War Cry: Nearby units take damage and have their damage reduced
- ?: Maybe like Earthquake but non-channeling
New upgrade system:
- Unique upgrades can be bought with gold and lumber from Neutral Shops. These unique upgrades may allow Thrall to harvest lumber from trees, add a ranged attack, permanent buffs or add an inventory to Thrall's companion and much more. Unique upgrades are mostly unlocked via optional quests.
Classic hero skills:
- Hero skill points are now used to upgrade special abilities like Crafting, Build Fire Pit, Cooking, Looting.
Crafting:
- Level 1: Allows Thrall to create items like Bandages or Mana Potions for gold and lumber
- Level 2: Allows Thrall to create items like Spears or temporary Shieldsfor gold and lumber
- Level 3: Allows Thrall to create items like Bombs for gold and lumber
Build Fire Pit:
- Level 1: Allows Thrall to build a Fire Pit that heals all nearby organic units. Burns lumber until there is no more lumber or put out.
- Level 2: The Fire Pit heals more, consumes less lumber.
- Level 3: The Fire Pit heals more, consumes less lumber.
Cooking:
Thrall automatically collects the carcass of a slain animal. The animal can be cooked at a Fire Pit to gain temporary bonuses. (pigs give strength, raccoons give intelligence, rabbits give agility)
- Level 1: Allows Thrall to cook pigs, raccoons and rabbits.
- Level 2: Furthermore allows Thrall to cook medium sized animals.
- Level 3: Furthermore allows Thrall to cook big sized animals.
Looting:
- Level 1: Increases amount of gold gained from slain enemies.
- Level 2: Further increases amount of gold gained from slain enemies.
- Level 3: Further increases amount of gold gained from slain enemies.
I am happy to say that most of these new features already have working prototypes.
