i've talked about the bella/reg/tom fucked up protege thesis with ivy and the vision is one in which both bella and reg have what the other one wants: reg is the wife and bella is the general. both would prefer it to be the other way around but neither of them know how to occupy the role that the other one so easily falls into. bella can never be as docile as regulus (and crucially crucially bella is not as young as regulus) and regulus isn't as competent as bella.
but i think adding to that, i love to explore the differences in the death eaters dynamics with voldy in the first war versus the second war. obviously in the first war voldemort isn't he-who-must-not-be-named, he's not like this immortal being with powers nobody knows. he's your average charismatic cult leader. he's in his 40s. there are people alive who remember going to school with him (one of them is literally regulus's dad lmao). in this way he's closer and more human and also less easy to fear. there has to be a process to him cultivating the worshipful adoration/fear that we see the death eaters in canon have towards him + i think regulus is the Perfect guinea pig for this process: young, pureblood, unloved, etc. i imagine tom telling regulus that he reminds him of himself when he was regulus's age and regulus just Glowing (except tom is lying -- he's too prideful to truly see himself in anyone else, he believes himself to be completely singular). and tom swans in and out of regulus's bedroom... there is a transgression of boundaries here that regulus initially does not trust and is not willing to entertain but this is the most careful and fervent attention he has ever received in his entire life...... how can he even begin to resist....... do you see the vision