Martin and I have been messing with savestates for years to do crazy things with mk64. The following will be an introduction to what we have done to map out what should be done.
The most obvious option is to do time trials and beat all the current WR's over at mariokart64.com This can be done but the result will not be considerably more impressive than what has already been accomplished over there. The only truely crazy techniques that could be used would be special shortcuts that are extremely difficult to do in real time. For example, 3/3 Mylestyle on LR Shortcut (SC), 6/6 SC on CM, and a more aggressive 3lap strategy on WS SC (like the fast lap (flap) strategy on all laps).
Those are potential intrests but, something that is even more interesting in my opinion is the 2-player vs. GP mode. In this mode, there is a timer for laps and the full race and other players can be used to allow shortcuts not possible otherwise. The following is a description of a subset of those SC's that Martin and I have discussed or made videos of (note, these videos were made using savestates and video editing equipment to compile the video based on savestates):
1. Royal Raceway:
Video:
http://www.people.virginia.edu/~avw6s/MarioKart/mk64TimeAttack/SavestateUsage/rry1870.avi
(Note, this video does not get the special warp SC that would allow for the path shown in the map)
2. Bowser Castle:
Video:
http://www.people.virginia.edu/~avw6s/MarioKart/mk64TimeAttack/SavestateUsage/BC_SC_Weatherton.avi
Also, I tried another angle but it seems just out of reach and the setup time may make it infeasible anyway:
http://www.people.virginia.edu/~avw6s/MarioKart/mk64TimeAttack/SavestateUsage/BC_SC_NoCigarWeatherton.avi
3. Banshee Boardwalk:
Video:
http://www.people.virginia.edu/~avw6s/MarioKart/mk64TimeAttack/SavestateUsage/BB_gpSC_Weatherton.avi
(Note: please forgive the low framerate here, the settings on the recording software were wrong and reproducing the shortcut proved extremely difficult)
4. Rainbow Road:
Video:
http://www.people.virginia.edu/~avw6s/MarioKart/mk64TimeAttack/SavestateUsage/RRd_GP_1328.wmv
5. Sherbet Lan:
Moving on to more difficult strategies, the orange path in this picture is theoretically possible and all parts of it have been tested but not compiled into one run. The strategy requires the use of the other computer players after getting the second player out to the island so that the first player can jump out to the island and jump off player two (who can get out by other means such as running into player one while they have a star)... the setup for this may make it only worthwhile for a fast lap but it would be a very exciting turn of events:
(and, yes, you can drive on the island in the middle with the Big Penguin)
I will add more as I find the time but this should be a good introduction into what is possible for the future. If you would like to see more, poke around at:
http://www.people.virginia.edu/~avw6s/MarioKart/mk64TimeAttack/
If you'd like to see a fun highlight real I compiled of all things kart (from WR videos to savestate play arounds) check out this:
http://www.people.virginia.edu/~avw6s/MarioKart/mk64TimeAttack/mk64_compilation_WeathertonFULL.wmv