My thoughts on this stem from the fact that for being such a flavor-driven process, it was forgotten that all 3 Stage BST 600 Pokemon only have one ability each plus a DW ability. This leaves us in a conundrum where we can either pull a Pupitar and have a DW Ability be the same as a regular ability, or we can be a little more subtle about it.
Since our prevos are now definitively Bug/Psychic, I think it would be much more useful to emphasize some element of that. Therefore my proposal is as follows:
Essentially, the prevos that have Weak Armor turn into No Guard Aurumoth, and the prevos with Forewarn turn into Weak Armor Aurumoth.
My logic here is that Weak Armor is actually a better gateway to No Guard because it is an ability that implies the Pokemon is open to dropping its defenses to gain an advantage. Aurumoth base forms that start with Forewarn are inherently more wary of foes but as they make the final evolution they are comfortable with a greater degree of risk. Illusion is kept as the DW Ability as a hidden trait of the species.
I think this works from both a flavor perspective and a competitive perspective, where if there were such a thing as a second stage Aurumoth, it might be useful to an Eviolite Pokemon to not want to deal with any truly negative effects.
[17:53] <&Deck_Knight> If I Cite and Prune CiteandPrune's post, what does that make me?
[17:54] <Birkal> a citeandprune cite and prunner
[17:54] <%DHR> O_o lol
[17:54] <+Mos_Quitoxe> Cite and Prune doesn't do enough of either
[17:55] <+Mos_Quitoxe> can we make him change it or force him to pay damages
[17:55] <&Deck_Knight> It would be a lot easier for him to Cite and Prune if we made him a mod.
[17:56] <&Deck_Knight> I delegate this task to Birkal.
[17:57] <Birkal> >:|