(^_^) retro is in the perception of who is making it really...
Retro for most of us is the CPC464, ZX-80, Eletron, etc...
whereas others would count NES & Master System as such...
I mean if you think about it Rich himself has mentioned the ST which is around as old as the SNES

which is alot newer.
Right now i think the Rules are games Pre-Playstation (before 1994) and N64

As most N64 games are actually on the SNES, i mean isn't that much of a leap.
If you look at it the classic games have basis on the console before in the Retro games...
Mario64 -> Mario 1 & 3 (on nes and snes)
Zelda64 -> Zelda III (on the snes)
Lylat Wars -> Starwing (snes)
etc...
as the idea is to take a Retro game and add upon the base giving it an original aspect, this means if i based my game on say Mario ... then the natural evolution would be Mario64. However not quite original - is an interesting choice.
Right now i'm leaning towards an Elité remake, cause in my own eyes that was THE classic Retro game - right up there with Pong, PacMan and Space Invaders.
And really if you wanted to make the ULTIMATE Retro game, Space Wars was the first Computer Game ever.
2 Ships in Space, you turn on the spot and can move forward and fire. The idea to destory the other ship.
Kinda funny how many people think that pong was the first, when it was simply the first home console/game
Got bored and started a mario64 clone just to see about the movement being possible quickly ... be interesting to find out
Tsu'va Oni Ni Jyuuko Fiori Sei Tau!
One block follows the suit ... the whole suit of blocks is the path ... what have you found?