I don't really dislike the oneshots at all, I think Natsu had enough team effort in the early series to build up him as a figher, And i really appreciate the power growth he had after Tartarous, I think some people can't get over the fact that he got power spiked so fast after the time skip. If you look at it, It isn't really that fast. For the majority of the manga, Natsu was weak and every arc he would get little to no improvement, His battle against bluenote after timeskip finally showed any signs of development regarding his power. He deserved this. That ain't my real problem tho, Mashima lost the consistency of writing Natsu overall. There used to be build ups to his maturity but now he doesn't have any. like i said in my previous posts, All characters developed romantic aspects but him, All of them had maturity in their default settings but him. He is like an early access game that the developers gave up on it. His character is lacking personality wise and i just hope to see any new aspect to his character besides being a moron and belligerent.
Ah you see I'm rather the opposite, in a series like this I don't tend to mind a lack of a conventional character arc or development of the character... principally because it's more of a light hearted story built around action and gags, as opposed to a serious drama or mystery etc. Natsu is pretty easy to like from the start, he's sort of the ideal friend for the main target audience, so he doesn't need to change in character or maturity levels all that much, 'growth' comes with facing up to new challenges and advancing his battle skills.
In terms of romance I don't think making his relationship with Lucy explicitly romantic would actually change all that much, it's heavily implied, and actually i think the way things have gone is in keeping with Natsu as a character. Conventional romance doesn't really fit who he is, but I think he does show Lucy he cares deeply about her in his own way (apart from the post Tartaros mess which is just bad writing from start to finish) ... I would note a lot of the pressure to marry IRL will come from the female partner in a relationship, so actually I'm more surprised Lucy hasn't tried harder on this front, but again the dynamic works as-is quite well. I agree to an extent it's weird to have secondary male leads like Gray & Gajeel develop their relationships while Natsu doesn't but I guess it's just down to personality variations. Natsu is less mature from the beginning...
I agree he's partially flanderised and screwed up Natsu's character in the latter part of the main series and into 100YQ, how stupid he is is really exaggerated compared to the earlier arcs of the series. He's constantly broken the rules of his magic with no consequences throughout the series, but it's become worse as he's been pushed to big undeserved wins like vs Sting & Rogue (2nd origin), The Spriggans, Zeref and the Dragon Gods. While it's good to see growth in power, it does need to stay balanced, or you end up with the Goku situation where he's so powerful you need to keep him out of the way while the other characters get to do their bit (e.g. travelling to Namek, suffering from a disease, or technically dead for the last 3 Dragon Ball Z arcs). Mashima has actually done a better job at keeping the power dynamic balanced, Gray and Erza (and even Lucy to an extent) haven't fallen behind like Krillin, Yamcha, Piccolo et al in Dragon Ball, or in the same way Naruto gradually became almost peerless in that series. The issue is he's done it via ambiguity, Natsu could be anything from about as strong as Erza and Gray, right up to easily the most powerful mage in the guild, depending on what the situation calls for. In a series like this which is centred around pairing off battles you do need at least a basic power scaling framework or it doesn't work.
I thought his huge power growth post-Tartaros was building up to him basically being the series' final boss, it's a shame that didn't happen as I think a series like this focused around a found family could have made that work so well. Instead it kind of devolved into oh, ok Natsu's here, no need to worry about what this bad guy's gonna do.
