I think in regards to the laserdiscs in MAME, there are 2 different things. First is when the preservation technology has reached the point where laserdiscs in general can be added to MAME and emulated. And the second is specific games (Dragons Lair and Space Ace and possibly others) where other issues (e.g. games being available in various ways) mean mamedev will choose not to support those games at all.