i will do my best to explain the methods to my neuvalin madness... i'll try to be brief but no promises. first off i'll establish i'm operating on the assumption that dvalin is indeed the anemo sovereign. and In My Mind he usually has a humanoid form in these scenarios... i do think he would take the opportunity to have one in order to be closer to venti + durin + the people of mondstadt, particularly if he could still switch back to draconic form at will... but how that humanoid form came to be is um. up to interpretation
i want them to meet when neuvillette comes to mondstadt to pass judgement upon barbatos and runs into dvalin on the way in (who has probably be sent with instructions to distract the hydro dragon until venti has found a suitable barrel to hide in for the next 3-5 business days). and i think they'd get along...
i really like the idea of neuvillette feeling great awe and longing about the draconic forms of the other sovereigns since his own was taken from him. it's not unlike durin gushing about how beautiful dvalin is, but with neuvi the desire is rooted in his own bodydysmorphia-esque issues; additionally i think the other sovereigns either aren't chill enough for this (apep) or aren't really able (poor azhdaha ass up in the earth) but dvalin would let neuvillette more intimately observe his draconic features. it's about neuvi running his hands over dvalin's horns and wings and tail in hesitant fascination (dude never touches anybody except for when the melusines want to hold his hand) and dvalin trying to remain composed about being touched and called beautiful and IM CRAZY IM CRAZY ok im calm.
i think dvalin's carefree attitude and relatively simple life of lazing around and enjoying the little things in the world (and only really locking in when mondstadt is in danger) could rub off on neuvillette and pull him in. i think meeting dvalin would bring to the surface his desire to simply connect with someone else of his kind and achieve peace in that way, and so for a few days neuvillette could lose himself in just doing gay dragon shit with this guy... but ultimately i don't think neuvillette could let himself fall into the complacency dvalin has. i think it'd scare him that for a moment he forgets what he came here for and what he has to return to. and they do fundamentally disagree on the archons/sovereigns issue... i don't think dvalin has any interest in reclaiming his power beyond what venti grants him to protect mondstadt, and he loves his archon and is entirely loyal to him (when he's of sound mind). what dvalin sees as peace + happiness, neuvillette sees as wilful ignorance + acceptance of his subjugation. i think they wouldn't be able to compromise either of their beliefs and venti probably just makes the whole thing even worse (plus probably has some feelings about what he might perceive as neuvi trying to ruin dvalin's happiness + turn him against venti) and it's all very yummy to meee... the fleeting joy hijacked by their clashing ideals... neuvillette going home having accomplished nothing and trying to refocus on what he believes is most important... dvalin probably doing whatever the dragon equivalent of eating a tub of ice cream and crying is while venti pats him on the head. etc.
i have more to say and i can certainly elaborate on specific facets of this but this is already super long i'm so sorry... umm yay gay dragons!