They can probably use their existing class features to accomplish these tasks.
While dandwiki has a poor reputation due to unbalanced homebrew, the examples you've provided (making a "holy hand grenade" and enhancing the abilities of their Steel Defender) seem like they should be possible to achieve by a standard artificer, using their existing class features and only minor house rules.
For the grenade, my first instinct would be to simply say "Sure. What spell are you casting?" The Magic of Artifice sidebar in the Artificer writeups say the following:
As an artificer, you use tools when you cast your spells. When describing your spellcasting, think about how you're using a tool to perform the spell effect. If you cast cure wounds using alchemist's supplies, you could be quickly producing a salve. If you cast it using tinker's tools, you might have a miniature mechanical spider that binds wounds. When you cast poison spray, you could fling foul chemicals or use a wand that spits venom. The effect of the spell is the same as for a spellcaster of any other class, but your method of spellcasting is special.
If they want to toss a "holy hand grenade" when they cast fire bolt or something, that'd be perfectly fine and within the aesthetics of the class.
Similarly, if they want to improve the abilities of their Steel Defender (or add entirely new abilities to it), there's a very simple option that they can take: using their Artificer Infusions to add those abilities to it with the Infuse Item ability. While the Steel Defender is a creature rather than an item, it might be possible for the artificer to infuse items that are then added onto it. For instance, it would likely be possible to infuse footwear for the Steel Defender that function as Boots of the Winding Path, or a searchlight that functions as Lamp of Revealing (via the Replicate Magic Item Infusion).
Whether or not you'd be willing to allow a Steel Guardian's natural weapons and armor to count as weapons and armor suitable for being Infused would require additional rulings, but I think that while they might not be allowed by RAW, allowing it probably wouldn't imbalance things; if they're using their Enhanced Weapon or Radiant Weapon to boost their Steel Defender's attack, they're not using it to boost their own attack, and its base AC is low enough that it's unlikely that boosting its AC marginally or giving it resistance to one type of energy would unbalance anything.
