Yes, "Mr. Do" would be nice and "Mr. Do's Castle" would be absolutely superb. I like this game even more, than the original "Mr. Do". But "Mr. Do's Castle" looks technically more complex to me. Whether it will work in fullspeed on the Amiga is the question? Perhaps it would work on AGA computers, but on ECS/OCS machines, i guess, it could be difficult. It would be really fantastic, though, because "Mr. Do's Castle" is really a great game.
And you know what? The game "Guzzler" (from Tehkan, 1983) would also be a great candidate for a Amiga port. It's a lot of fun and not too complicated. And besides, there are only very few home-ports of this Arcade game available (it had a Sega SG-1000 port and recently, an MSX port of the game was released). Just some suggestions. :-)