1) A Halo racer (not just the race gametype - a full-on racing game set in the Halo universe)

or

2) A Smash Bothers-style Halo (i.e. Keyes, Johnson, Chief, and Cortana duking it out on a 2D multiplayer map)

Why would your choice work better?

I think I'd enjoy the Smash Brothers type of Halo game more-so than a racer. I think it would have a bigger marketing potential too. Hell, I would think with the hardware we have these days the player count could be increased and a large number of gametypes could be included (2-8 players, FFA, team games [slayer AND objective]), too. I haven't played smash in 12 years so I'm guessing Nintendo's already upped the player count and gametype options - but what do I know?

*I also never see either of these two coming to fruition in my lifetime unless the project was outsourced completely. But it's fun to speculate!