What you commented is completely correct under the assumption that you want to write a series that is 100% like the original series. However, not everyone wants to do that. Both I (with my ReInitiated series) and Systemcat (with the Iron and Steel universe) write series that are more serious in nature, though with the classic Gadget humor mixed in.
That said, there are rules to this type of fic as well, so I will amend your points to fit:
* Creating your own adventure: I have nothing to add. What you covered fits quite a few stories, my own included.
* How it Happened: Same here. Having played with this part myself, I'd agree with your pointers.
* Use of any version of the series: I agree with you to a point. Use of all the series and blends of each series is good, but strictly following formula is not always necessary. Not even canon did that (compare the Classic series, the movies, and the 2015 series. They're all kind of different.)
I'm going to expand out to say that you do not need to adhere completely to the formula of the series. Unless you are writing a purely humor fic, a repeating, Status Quo is god fic will get rather boring, rather fast. As someone who loves the Inspector Gadget characters, but am horrible at pure comedy, I cannot do this style of fic. That's not to say these are bad; I've seen some brilliant fics in this style. I'm only saying that some people, myself included, are not skilled in this area.
Instead, I take elements from all series and blend them together in a way that allows for a bit more continuity. Sure, missions are the core of every story, but every mission changes things a little bit. Injuries endured on one mission will be felt later and lessons learned in one episode will be remembered later on. This allows for character growth and development as time goes on.
OC (Original Characters): Here's where I'm going to disagree with you a bit, but I'll start with what I agree.
Yes, I don't think and additional (blood) family members should be added to the Gadget clan. If there were other blood relatives, they would have been mentioned. But the fact that Gadget and Penny are referred to as each other's only family in canon makes additional members very unlikely and improbable. Maybe a skilled author could do it well, so I won't tell someone not to write one, but I'll just say I can't think of a situation done right.
One step with OCs that I will add is I'm not fond of fics that star OCs. Even if the canon characters are there on the side, the fact of the matter is that few people want to see an OC in the spotlight. The readers are attached to the canon characters, not some out of the blue newbie they've never seen before. An OC protagonist in a fanfic is a quick way for me to dismiss the fic. Once again, this isn't to say that all fics of this type are bad, just that they don't interest me.
Now here's where my disagreement starts. Your rules for OC friends and MAD agents (one episode a piece) only works for pure comedy. After all, if someone wants to write a story with more drama, stakes, and character development, reoccurring characters, both allies and enemies, are a must. People in real life actually interact with each other on a repeated basis, thus so too should characters reoccur. I will agree that they should not become more important than the main characters, but coming in again and again, as well as developing them alongside the protagonists, helps the story, not hurts it.
Finally, I will disagree with you vehemently on love interests, even OC love interests. Now, I can see where you are coming from. Too many stories devolve into a character and their love interest gushing over each other, and doing pretty much nothing else. That sort of story is bad, but a love interest does not instantly mean this type of story.
All love interests, even OC ones, can be good, if you follow these rules:
1. Think about why this relationship is happening. Why do these two want to be together? Do they share interests? What to they sit and talk about that IS NOT ROMANCE RELATED. Do they feel safe and comfortable in each others' presence, and is there a reason for them to feel this way?
2. No love at first sight. This just isn't real. True, there are feelings of attraction that happen at first sight, but love grows slow and develops over time. Feelings of attraction will come and go, fading over time, but true love is a willed decision made over a long time of knowing a person. Make sure your characters' relationship is not built on gushy feelings.
3. The romance does not engulf everything else. If you are writing a romance story, than focusing on the romance is OK. However, is you are writing anything else, try to keep the romance subdued. Real people in real life have romances yet also hold jobs and have hobbies that have nothing to do with the romance. Make sure your characters' love life does not rule their overall life.
Now a story without love interests is fine, and can be very good, too, but I find the idea that all romance is always terrible to be flawed.
Now to touch on some rules to writing darker, more serious stories in specific:
* Make sure it is a series that is open to the more serious atmosphere: I think IG is a series that is open to a more serious style (the original had a scene where three men and a dog were almost murdered with giant ice-cutting blades), but not every series is like this. I would recommend putting plans of Teletubbies: The Reckoning or The Great Bikini Bottom Massacre in the garbage can.
* Make sure you don't lose the original humor: Just because a series is dark doesn't mean it can't be humorous. Think of series like Spectacular Spider-Man or Batman the Animated Series that had good, serious feels to them, but had laugh out loud scenes.
This rule is especially important for a series like IG, which originated as a comedy. Yes, you can make the series have more severe stakes, but make sure there's still levity. Cracking jokes now and then is a great way to make sure your readers are not overwhelmed by the darkness.
* Make the darkness come from a source that makes sense: This is the biggest crime most IG darker series have. They think that to make the series more serious, they need to turn Gadget into some snarling melancholic counting down the days till he dies or Penny into some neurotic recluse who had an emotional breakdown as a teenager. (Yes, both of those are real interpretations I've seen.)
Instead, think of a better source for danger: the villains. After all, they are the ones supposed to be threatening in the first place. Keep the heroes as shining beacons of hope and optimism (as well as some humor); after all, the audience is supposed to like them! Instead, make your villains be the sources of darkness that need to be stopped.
Well, that's all I have to say for now. Maybe I'll add more later.