I have been seriously contemplating adding multiplayer (hotseat/split screen only) capabilities to all of my games for the January re-release, these are my thoughts:

Blow Stuff Up

Two modes, a co-operative one (the two players share one timer, though one that ticks twice as fast) or competitive (each player has their own timer). The competitive mode could be quite interesting, as players would not just be trying to kill each other, but will fight over points to stay in the game.

All I really need is a Mesh to represent the players! Maybe I can use the OGRE robot model..

Derelict

I think, just adding a second player to the game wouldn't really work, as it'd be much more difficult to manage the team of marines on a small screen.

What I have in mind is the addition of two new roles that are only playable in co-operative mode. Player one would control a Ranger, who carries a grenade launcher in addition to a motion sensor. Player two would control a Combat Medical Technician, who can hack, repair, heal and walk through dangerous areas. Each role would lack the capabilities to complete the game solo, but they would be able to complete it as a team. There would be no AI marines in this mode.

Firestart

This one may be too complicated to bother with, for instance there are issues surrounding the events that only trigger when the player is in a certain location. It might work out well but I'll put this one way down on the priority list.

Heart of Ice

Since there's already two playable characters in the game, and the action takes place on a single screen, this should be straight forward. A harder difficultly level might need to be added to compensate for the extra killing power the second player provides.

The Kiwi's Tale

It shouldn't be that hard to just add a second Kiwi to the game (Perhaps one in a nice shade of Trademe blue) The big problem will be when the players are so far apart from each other that one is off screen, which was certainly annoying in the Sonic games and could be more so here. Split screen, either on permanently or when the two players are apart may be possible.

WitchBlaster

Should be the easiest of the bunch to implement multiplayer. Though I may have to ramp up the difficulty level in two player mode to compensate.