Portal in 2d for Amiga 68k
Sideview plattformer with normal gravity.
You control an avatar, not first person view.
Aiming could perhaps be done with a version of the turricane lasso.
Hold button rotates arm. Release shoots off a bolt. Another click places the portal on the wall.
A jump into one of the portals carries with it the speed and arc when exiting the other portal.
Difficult logic in puzzles to advance and precision jumping. (but not to difficult). :-)
I think 2d Portal could be very fun and addictive.
It would be great if it was made in way so that levels could be added in an easy way so there was no end to the challenge.
There's an ASCII 2D Portal game that's similar to what you have described, despite the graphics it's well made. There's a port for OS4, since it requires ncurses which is available for OS 3.x it might be possible to port it to 68k systems as well:
http://aminet.net/package/game/actio/asciiportal_os4Varthall