I'm quoting you, not because I necessarily agree/disagree with your post, but because you bring up things I want to talk about.
I agree that memorable characters are forged in play, not designed on paper.
I don't see an either/or here. PCs are not
either forged in play
or designed on paper, they are all
both. This reminds me of the Stormwind Fallacy.
For me, both characterisation (fluff) and game mechanics (crunch) should complement one another, ideas bouncing between the two during character creation until you are finally happy with both and are ready to start play.
Will it be interesting? To other people?
In my opinion, 'interesting to other people' should not be the goal of character design. Sure, it's nice if other players think your PC is interesting, but it's not the point of RPGs.
For me, the point is to enjoy the experience of playing; but what's the surest way to that goal? First, THE most important thing is that you think your OWN character is 'cool'. YMMV on what
you think is 'cool', but it's what
you think of your
own PC that makes you
want to play this PC over and over again.
And sure, it's
nice if
other players also think that
your PC is 'cool', but it's not essential. What is essential for
them is that they think that
their PC is cool!
Second, for me, it is essential that I'm good at what I do. I really hate playing a warrior who can't hit a barn door or a caster whose spells have no effect or a skill monkey who fails rolls more often than they succeed. The whole game would be one frustration and disappointment after the other. This means I have to pay attention the the PC's game mechanics so that a.) it can mechanically achieve success doing what it is designed to do, and b.) that what it achieves mechanically matches the concept that I am going for!
If I design a PC whose mechanics make it a superb archer for its level, I'd be unhappy if the
concept of the character was to be the best caster I could be! How many times have I seen players deliberately choosing to play, say, the party face whose design simply doesn't do the job.
They get frustrated because whenever they try to do what they were designed to do, it fails! The rest of the party feel frustrated too.
As an example of how I design to achieve these goals, I'm in the middle of conceptualising a replacement for my current PC in case of his untimely death. It usually takes me a couple of weeks from beginning to end to come up with a PC whose fluff and crunch are 'cool' enough for me, so I want to get a head start. I can't complete this PC though, because I don't yet know what level my current PC will be if and when he dies (and therefore my replacement's level), and I obviously cannot roll the replacement's stats yet.
I want to try the hexblade. My current PC is the only proper melee warrior in our party: Bar 1/War (fiendish bladepact) 5 right now, whose role is that of a melee striker (no
eldritch blast for me!), alongside a rogue (thief), a Raven Queen tomelock polymorphed into a cat (don't ask!), and an arcane archer. This means that my replacement PC must also be mainly melee. Fortunately for me, this is my favourite kind of PC to play.
Looking at the hexblade rules I come across the 6th level ability to create a spectre (that lasts until the end of my next long rest) from someone I just killed. I'm not really sure how that fits in, so I think about it for a couple of days. Suddenly, I get inspired! (this is my usual process, BTW) A Valkyrie! A Chooser of the Slain! How cool is that? Now I can fluff that weird spectre ability, and build my whole PC not around that mechanical ability but around the whole Valkyrie/Chooser of the Slain concept!
Great! But I still-and this is the part that is relevant to this thread-I still have to make sure that the mechanics support that concept. There are various ways I could do this that would still make sense, and I could pick any of them. But there are many
more ways that would not make sense at all!
So, I'm going to be an aasimar. I know half-elf would be mechanically better, but aasimar are still a fair match for what I want to do mechanically, and (crucially) it matches the concept of a divine spark necessary to capture the idea of Odin's semi-divine Choosers of the Slain.
Class-wise, I can either go full hexblade or I can multi-class with vengeance paladin. Either would work for the concept, and either would produce a mechanically sound melee specialist with sufficient other stuff which would both support the concept and have utility in play.
For example, I'm imaging The Ride of the Valkyries as horses galloping through the air. Simples! At Pal 5 I can cast
find steed to get a warhorse which I will fluff as on offspring of Odin's own eight-legged horse, Sleipnir. I can even say that when you see it gallop out of the corner of your eye it seems like it has eight legs, but when you turn to check it has the standard four. At War 5 I will choose
fly, and when I cast it on myself it will also affect the horse. And there you have a horse galloping through the air!
I think this is cool. I hope the other do too, but that is not the point.
Will they find her 'memorable' or 'interesting'? I don't know. I
cannot know until I actually play her. But 'memorable/interesting' is not really the point, it's just a nice bonus if it turns out that the other players think so.
Still one thing I find that makes it easier for me is to pick contradictory aspects in the Background. At first it may sound like a recipe for disaster, but people are contradictory all the time. The hero that is a coward right up until the moment he isn't is all the more memorable.
Having a flaw in direct contradiction of your Ideal or Bond becomes a built in challenge to overcome. Conversely the realization that the flaw will make your Ideal unreachable could be an interesting shift the other way.
Or just keep it simple. Having both: I love a good insult, even one directed at me and I blow up at the slightest insult as Personality Traits can lead to some great fun.
I agree with you here. I
hate playing cardboard cutout stereotypes!
A recent 5e PCs of mine is a berserker barbarian with the noble background, who is from Waterdeep, is super-civilised, and who hasn't wandered around in a fur loincloth in his life! He does have some anger management issues though...!
Another concept is a zealot barbarian (again, not a loincloth in sight!) based on the WildStorm comics character Zealot. It may be hard to believe, but the name is just a coincidence.

She has the acolyte background, and was left on the steps of a temple of Tyr and raised by them. She was taught a secret fighting style involving going into a kind of battle-trance (re-fluffed Rage), and bases her actions around the sacred precepts of Tyr. She is always serious, never smiles, and personifies the recent 'Resting Bitch Face' meme, even though she is compassionate as well as fierce in battle.