Could I ask you to introduce new classes/races/subraces/domains as well?
You could use Eye of Gruumsh and the other class(disciple/monk of sth) that was partially developed by Bioware (which shadooow discovered and included in his patch) or even include some classes from PRC (those would need to be balanced though).
I'd have to look at Eye of Gruumsh and anything else potentially being added, class-wise. It would definitely be a lower priority.
What new races/subraces would even be added? The main problem I have with those is that they're usually code for "I want more power! Er, I mean I think we need a Half-Orc subrace that gains 4 str at the cost of 2 wis/2 cha. Balanced."
By domains you mean cleric domains? What would even be added?
Truly its not that you actually need any henchmen in any OCs. Within 5 minutes of any OC game launch I see the inevitable "how to solo" thread in a forum ~ LOL. It was true of NWN (1 and 2) BG (1 -3) KOTOR and Dragon Age... and many others..
I wasn't even trying to spark a discussion about soloing, I was more wondering what particular benefit an arcane caster would be considered "essential" for compared to a fighter, rogue, or cleric (or whatever). I mean, if the arcane caster does nothing but summon (more powerful) creatures and snipe with a crossbow, I'm guessing people wouldn't be thrilled (even if the mage's summons were so powerful they were better than a fighter). They probably expect AoE spells and/or IGMS and/or things like dispels/spell breaches. But what ratio?
This was in fact one of the single criticisms of HoTU and frankly the loudest criticisms of SoU.
Lack of a pure cleric/mage specifically or lack of companions overall?
As I suggested (though other may have many better ideas)...
If anything I'd be more concerned about writing/integrating them well enough than the idea itself. Like rogueknight said, I wouldn't want a situation where a default companion is better lore/story/dialogue wise but people feel compelled to bring a more powerful "added" companion.
HotU is where the basic crafting was added (weapons, armor), and that might need a bit of focus in order to make sense in the module. For example, if PC crafted weapons were created with an item property (such as an on-hit) - one that would not be part of the forge upgrades - then PCs in chapter 2 and 3 can upgrade the weapons they crafted to add more variety.
I'm not sure if it's even worth trying to make crafting good/useful, frankly.