I've noticed a nice approach done by FFG with DungeonQuest having Miniatures compatible with other games from the same universe. You can use the heroes that comes with DQ in Descent, Runebound and Runewars games! So, I think the investigators in Mansions of Madness to be investigators we know from Arkham Horror games and thus we can use the Pre-Painted miniatures in Mansions of Madness as well.
I would also expect some Investigator cards for Arkham Horror IF they introduce new ones, just like DungeonQuest having hero cards for Descent, Runebound and Runewars. The bad thing about it is that they are not going to be pre-painted