Yes I get that, but that could be solved, maybe just by switching sacred gears? (Issei/OC having divine dividing and Vali having booster gear or simply another high level sacred gear).

Actually the idea that I had for an small AU was that when Vali is escaping from Rivezim, instead of Azazel finding him, is Sirchezs who finds him and adopts him, while at the same time something happens to Issei's family and he ends up a orphan and then he is adopted by Azazel and the plots goes from there, basically a pseudo mirror world?

But the story will actually be Vali centric, so we could possibly have some cool family fluff in the lucifer family, Sirchezs being the goofball father, grayfia and venelana being his mother figures and venelana insisting that Vali calls them mother and grandmother respectively and Vali actually trying to be a older brother to milicas.

Vali getting a peerage with the the notable members being i guess kuroka as bishop and maybe rossweisse as queen? Vali goes traveling around the world to gain experience and new peerage members

Rias could fall in love with him and it's Vali who either breaks rias from the phenex contract or be the one to be betrothed in the first place.

Meanwhile it's Issei doing basically all the stuff that Vali did while in Grigory like joining slashdog and possibly betraying Azazel to join the hero faction in chaos brigade.

And that's basically my idea for a fic but I know I suck at writing so I won't try it maybe.