This (d20 + bonus) was the more interesting idea i had. It would be more intuitive,as you said. But the reason that i didn't go that way was the retrocompatibility: since the Elfgame need to use the bestiary from BX/retroclones, it would create two rules for the same thing (PC would use d20+bonus, monsters would use the standard procedure)
The whole idea was to mantain the Elfgame able to be played with any and all material released in the past 40-50 years