Alright, as self-appointed dragon expert of the unofficial Chaosium discord (a position which I am definitely not qualified for), here is my opinion on dragonewt PCs: don't.
Or at least not traditional dragonnewts. A barbarian dragonewt (aka one which is no longer part of dragonewt society and cannot resurrect/evolve) can be fun to play, as they still retain some dragonewt features while also being able to adopt other customs and magics. How you would go about determining runes and passions for them is a mystery to me though. Perhaps they acquired a connection to one or more runes resulting in their departure from the path? In any case they'd probably still be an odd duck to play.
Perhaps you could play an orthodox dragonewt temporarily, one who is on a mission from their temporal ruler. If so I'd suggest imposing some restrictions on their behavior given by their ruler, to better integrate with mortals, and also focusing on the threat of them losing the path of they become too involved in the world. Plenty of odd, random behaviors and bizarre rituals should be enacted by the player, and then and the GM should discuss their dragonnewt's mission and restrictions/obligations in private.
In either case I'd recommend you keep player characters to the Crested caste. That will both keep them from completely overshadowing the party in combat with higher castes' dragon magics and hightened physical abilities, and it's more reasonable that they would have fallen/be on a dangerous mission like that. Also if they're barbarians they can still learn non-dragon magic if they're created.
Anyways, YGMV. These are just my suggestions if you want to try and sick to "canon" (at least I hope this is acceptable within canon), but you could always change up how dragonewts work or interact with the world if it suits your game.