So, I have been thinking. Abraxas Malfoy died of Dragon Pox and Lucius Malfoy came into power in his family, with a steady hand over the pockets over Ministry officials. It seems like a coincidence, but is it really?
Is it possible that Abraxas' death was just a ploy of the Dark Lord? We can't know for certain that Abraxas - long-standing Pure-Blood Patriarch - would have sided with the Dark Lord during the war. He may have been sympathetic to the principales, but a number of people were in those times; it didn't mean they were willing to KILL Muggles/Muggleborns - they just thought they were superior. Is it possible that the Dark Lord - a potions master in his own right, having developed the Drink of Despair that protected the Horcrux in the cave - could have poisoned him, in an effort to get a more vulnerable, easily swayed man - being Abraxas' young son - into his ranks?
I obviously know nothing for certain, but my best friend and I have decided, in our personal head-canon, that Lucius murdered his father himself (potions, as you said), either because he objected to Narcissa (the sister of that blood-traitor!) or because he was abusive towards precious little Draco.
(Lucius seems the type to have potentially had a violently-overbearing father whom he despised. Possibly because we just can't see Lucius ever being "cuddly" toward anyone but Cissy and Draco. To go along with that, in our head-canon, Abraxas killed his wife over some transgression while Lucius was in his first years of school. So no real warm fuzzies between father and son at all.)
I do like the idea of Voldy doing him in to secure the Malfoys' influence on his behalf, though. That sounds all kinds of fun, whether Lucius was in on it or whether the Dark Lord went over his head--I like that. Oh, I can just see Lucius: "Oh really my lord? Well, if you think it best, of course. He's a canny old bird, though, you'll have to be subtle. Best of luck. Say, Bella, is there any more tea?"
I mean, wizards generally live to be really old (except it seems in the pure-blood families...hmm, suspicious?), and Dragon Pox is not, apparently, often a killer in modern times...so something happened. Although Slughorn didn't seem surprised...maybe Abraxas was just really old before he managed to get himself an heir?
Anyway yeah, no idea. But I like yours! I always prefer the ideas that carry more drama. Especially for those wacky pure-bloods.