Amrûnril
Hero
Honestly a lot of the existing ones are just a little too restirctive which means there's a lot of stuff that's almost doable with the existing rules but not quite...
For exmples:
The Armored Artificer is stuck with gauntlets as weapons. You can't make a magic hammer or whatever without needing to max out str rather than int which means you lose out on a lot of core features... and have to set aside a lot of the power of the subclass. But as written you can be Iron Man but not Steel.
The artillerist is so much better at temporary hit points than not using that one option feels like nerfing yourself. And the walking cannon is better than the hand-held one... so playing a gunslinger type is working against the features.
Battlesmith ties up so much power in the robopuppy that playing the subclass as a sword-artificer seems stupid. If the pet was optional, this would be the best swordmage in 5e, better even than the hexblade.
All of these could be tweaked with more optional swap-outs to make them vastly more versatile.
Very much agree with regard to the Armorer. This could be easily be expanded into a generic/flexible warrior subclass if it gave a few alternatives to the Iron Man inspired armor abilities. With Armorer expanded to fill more warrior concepts, I think Battlesmith would actually be fine staying as the pet/construct focused subclass.
Meanwhile, the Alchemist seems like it's in a good place thematically, but its mechanical weakness leaves it unappealing to a lot of players. The Artificer is balanced around having strong subclass abilities to make up for being the the only class that doesn't get extra attack, 9 level spellcasting, or sneak attack+uncanny dodge as core abilities. Elixers and proficiency to damage/healing aren't enough to fill this gap, even though they'd be great subclass abilities in another class.