This is an intriguing development.. I just had a lightbulb moment.
If theplottwist and I merged our Legends timelines:
LOS>MOF>LOS2>>>Legends>SCIV>>>>>>>>OOS>LOD>64>VK
Simon from LOS was Sonia's Grandfather from the Legends prologue.
Reason being is that she may not be a Belmont by blood, which is why he isn't her father.
Sonia defeats Dracula (Gabriel/ Vlad) had a child with Alucard (Trevor) who she names "Simon" after her Grandfather.
This is the same Simon from SCIV, who defeats Dracula during the middle ages "hundreds of years" prior to Richter Belmont.
OOS' Desmond defeats the "guardian of Dracula's sleeping spirit, thus Castlevania doesn't crumble. The Castle remains active.
Desmond has at least one son and one daughter - the daughter begins the Schneider lineage off the existing Belmont Bloodline. Therefore, Desmond being Reinhardt Schneider's ancestor (red hair and all).
Desmond's son carries the Belmont name, however, the Schneider's were either more well skilled to use the Vampire Killer, or they were passed it for another reason. (like on the main timeline)
Cornell defeats the real but not yet fully resurrected Dracula who absorbs his Lycan form, resurrecting as his first born child, Malus.
The guardian of Dracula's sleeping spirit is defeated by Reinhardt and Carrie (like Richter/ Maria at the end of SOTN), they then defeat the real Dracula (aka Malus)
Richter Belmont is the last Belmont descended from Gabriel to defeat Dracula (who now looks like Malus due to his previous incarnation).
FIN
There is a glaring issue here, however, and that's why I asked Enric for the date where LoS2's intro happens.
In LoS2's intro, Gabriel is struck with the Crissaegrim, and sleeps for hundreds of years until 2057; He couldn't have battled anyone else until his appearance in this year. Some sources say that Alucard stabbed Gabriel in the 1400s, and if I'm not mistaken, the knight scrolls hint at that.
Depending on when he was struck during the 1400s, Sonia could've fought him (and could've been the first human to actually defeat him - as her story officialy goes - since he was not truly defeated by Simon in Mirror of Fate) and THEN he revived later to be struck by the Crissaegrim. But it's important that this date be anywhere AFTER 1450, where Legends takes place.
In the LoS's timeline, LoS2 is obligatorily the last battle. Anything that happens between Crissaegrim and LoS2 must not include Dracula to work.
So, it would go like this:
LoS >> MoF >> Legends >> LoS2's intro >>>>>>>>>>>>>>>>>>>>> LoS2
Also, important to note: In this bizarre headcanon of mine, I inserted Sonia PURELY because the last Belmont to face Gabriel is Victor Belmont. Victor is Sonia's descendant in the cancelled Castlevania: Resurrection

But, as stated above, there are many issues with the idea. Not only the possible incest, but also the fact that the game flat out states that this is the beginning of the Belmont legend (when, in this hypothesis, it's not) and that the child in Sonia's arms will battle Dracula in the future and be hailed as a hero (when this wouldn't be possible due to Dracula, in this hypothesis, being put to sleep by the Crissaegrim - unless this child is actually Roland de Ronceval, who died fighting Dracula BUT still got considered a hero throughout history, though he's not a Belmont).
I entertain it more out of experimentation, because I'm writting my own "Branching Timeline Hypothesis" to introduce in my project as a means to explain Dracula's presence and the involvement of time travellers.
EDIT: OK, I'm not sure if this new info destroys the entire hypothesis (probably does), but LoS2's intro takes place in the year 1547.
If Sonia existed in this timeline, then she did almost 100 years before this.