However since Arthur getting Dumnonia is reliant on him taking it from whoever inherits it after Morgan dies, since Arthur is a bastard he doesn't get it and ends up just holding a tiny holding which means his chances are low of staging a take over. There should be an event where if Morgan where to die without an heir (especially if Arthur is still regent), Arthur is given a chance to take the crown for himself (maybe it even turns him into a legitimate bastard?), that would at least give Arthur a fair chance of rising in most games without just dying in obscurity like he normally does.
I'm going to assume you mean Mordred here.
Well, in the books, this is pretty much completely antithetical to Arthur's character. He never wanted the throne (despite the desperate pleas of pretty much everyone else), is willing to give the entire kingdom for "peace", and even when Mordred was thought to be dead he only pushed for his son Gwydre to be king (and even then probably only because Gereint, who would have been the legitimate heir after Mordred, was long dead). Dying in obscurity sounds a lot like what Arthur probably would have done if the events leading up to Camlann had never happened - it's all he ever wanted.
Honestly, I do find that Arthur actually does gain the throne a fair amount of the time when Mordred dies - he's loved, so when he revolts, everyone revolts with him. There is actually already an event that turns Arthur into a legitimate bastard if he becomes King, IIRC.