If you could have one prototype of a game...

[vestcoat]

Ultima - Savage Empire for SNES
Bonk RPG

[Black Tiger]

I'd like to try the earlier shown, nice looking stylized Bonk RPG, which reminded me of a Phantasy Star.

I'm glad that the bad-NES-game-looking Bonk RPG which was shown in greater detail was never released.

[Tatsujin]

could Xexex been adapted to PCE? was there a proto at one time? damn that would have been interesteing to see.

Here are most of the abandoned, stopped, unreleased, protos etc. listed.

http://www.pcengine.jp/a71.htm

Xexex unfortunately is not listed, but I have read back in the days, that there were plans for a PCE port. So I guess that there is no actual proto of it, but I wish there is.

Even the JPN Wiki has a mention about it:

http://ja.wikipedia.org/wiki/XEXEX

PCエンジンへの移植計画も一時期存在し、CS開発チームも積極的だったが、困難が多く中止となった。

"At a time there were plans for a pc engine port, and even the CS developement team was quite active, due to certain difficulties it was ceased."

[RR1980]

that's really neat that RPG Genjin was supposed to be on a hucard.

[Ninja Spirit]

PC Engine Batman, when it was originally proposed as a side scrolling platformer seen in Famitsu magazine with Batman sporting the classic grey and blue colors.

[liquitek]

Maybe a bit lame in 2011 but in 1998/1999 Thrill Kill for PS was one that I was really po'ed about not being able to buy.  Stupid EA 
So getting a good beta would have been a real treat.

2nd the MotM 2 as well...

[Gao]

Match of the Millenium 2 for Neo Geo Pocket.

This.  I'd also like to see Garou 2 for the Neo Geo.  And it probably would barely be playable, but I'd like to see how far they got with Mario FX for the SNES.