Is it possible to have an editor?

Not sure what exactly you're looking for, but if you choose a household while still in manage worlds and click the pencil instead of the play button, you're editing that household. You can also go to manage households to merge households.

Lots similarly can be entered directly into build mode, and when you do so you have an unlimited budget and no household inventory.

It would be nice, however, if family relationships could be set to any Sim in the save file, rather than just to others within the household. They already retain the relationship when moved out, so it's just a matter of being able to create it...

Ah. I'm referring to adding a more in-depth info about the sims of that household, like their career and the level they would start as. For example, I am trying to move Iggy Pancakes (toddler) from the Maxis-made upload on the Gallery (where the Pancakes Sims-style T-shirts). But that upload's Bob and Eliza Pancakes are blank slates compared to the pre-made townie version, in which they have a set of skills already leveled up, and a job. Bob Pancakes being a Caterer (rank/tier 3 of the Culinary career). If I choose to just use the new Bob and Eliza, instead of having to "play" the household, get their careers, either work his up to Caterer (or cheat it), I was hoping we could just edit it via the sim's info rather than having to do a whole playthrough. This would be useful to those like to make save templates.

Another one is the relationship of the sims. It's easy and known that married sims would have a green friendship level up as well as the pink romantic bar. But what about those I just wanted to make as dating, or starting off as boyfriend and girlfriend? Also, I mentioned having to be able to link familial relationships, like how Johnny Zest still has no blood relation to the Landgraab, as he isn't part of the Landgraab family tree. Even if he's just disowned by his parents, it's still a bit... immersion-breaking to know Johnny has no family tree. In which, yes, I agree with you. I hope they add this in the future before they finish up the very game and move onto prioritizing the Sims 5 (Project Rene). I fear the Sims 4 may have only a couple or few years left before being left with its final update, and some features, systems, and mechanics would still be lacking. But I digress... ^_^;;

Though, I'm not certain whether a "save editor" or world editor has been done on a Sims game. I do hope they would make one, like how they make pre-made townies so it's easier to manage them.

Ah that makes sense! Like the way going through the story start of a Sim gives pre-existing skill levels, but with a bit more control… probably would have to be a cheat code because it seems a little silly to allow people to create maxed skills without them, but I definitely see the point.

And then another layer on top of that would be setting Sims relationship with each other - I think that actually does exist as a cheat code though?

Yes. But as for it being a cheat, I was hoping for it be on a menu instead of having to play the household. Like its own CAS-type of... "room" or mode? And yeah, maxing skills would be considered a cheat, but that would depend on who utilizes it. I, myself, would just bump a couple of skills to 2-3 levels like other pre-made townies, as well as give them careers that can be either available for the players, or NPC careers like being a food stall vendor, or a street performer, or one of the professors in Britechester and/or Foxbury. The latter idea being difficult to configure as there's a lot of steps to do just for them to have NPC careers and function. Instead, if it's a permitted job, it can be toggled easily through, as I've mentioned, some sort of menu or mode similar to Create-A-Sim.

What you're asking for reminds me of SimPE for Sims 2. It wasn't anything that EA was involved with, it was a fan made program, but it had a lot of editing features for so many different aspects of the game. You could edit everything from objects, to sims, to neighborhoods, and so many things. It's a shame something like that doesn't exist for Sims 4, because I remember using SimPE like crazy.

I think the closest thing you will find to that in Sims 4 would be MCCC. It has a lot of sim editing options, some of which are things you mentioned. So if you play on PC, maybe give MCCC a try.

Now that would've been interesting, yeah! We have S4PE, but I don't know to what extent it does to the game. I have MCCC as well, but they do not adjust the relationship of the sims outside familial (spouse, siblings, children, etc.). It's almost a decade and no one has yet to plan and create such a program. It'd be very, very useful indeed. If you don't know about S4PE, perhaps you could check it out? I don't know what it does exactly yet, but if it's good, I'll use it as well. ^_^