Why must it be illegitimate? 'tis entirely plausible he fathered a daughter after the events of DoS, or before his fight with Dracula at the turn of the century. The latter couldst be explained by his amnesia and a reunion following the events of AoS, training of the child during latter stages, and the handing o'er of the whip ere the story begins, one wouldst assume. The former, well....the Belmonts art a family of legend... >;)P
As for which way to send the story, methinks the decision shouldst be left to the author, as both options couldst lend most admirably towards a good plot. The "Somacula" plot couldst be looked at in a redemptive manner, both for the Belmont line and Soma himself, and the story plot allows freedom to spin rivalry 'twixt Soma and the protagonist as power is displaced throughout.
Either way, I am playing through thy first offering at the moment, and am rather impressed for once, so I am pleased to hear of thine idea to create another offering.