The main problem that I would expect is that you'd probably get disagreements with the computers readings, which would start messing with the EFI mapping.
First problem is - does the TPS you're looking at actually use resistance on the signal to measure or does it not (although I believe the VAST majority of TPS modules do work that way). After that, is the question of, does your TPS increase or decrease resistance as it's moved to WOT?
Example would be that the signal is sending at 5vdc and returning at 3vdc at closed throttle. Then, at WOT, it returns 5vdc. In this case resistance is reduced with throttle travel therefore adding a variable resistor would result in a backward affect.
The next issue is that (assuming resistance is increased with WOT) adding a variable resistor set to 25k Ohm, you'll have this resistance at all points of throttle position. Meaning that if the TPS supplies 45k Ohm, and you add another 25k Ohm, that the signal will fall outside the ECU's expected parameters when the throttle's closed.
As a final note, the computers can have multiple sensors for throttle action. So if the TPS is reading something, the pedal position sensor is reading something else, this will create a conflict for the computer. That's why I say that you'd minimally get EFI issues I think.
I wrote all that as you guys were discussing in the comments. So to update it to match that information, it still depends on which way the sensor works, and you'll still face the problem as noted above with italics.
ADDED NOTE: These are what I'd call the "foreseeable" problems. There are other possible issues that you'd run into, and there are ways of building much more complex circuits to mitigate those issues as well.
Ultimately, it seems like what you're looking to handle would require some pretty advanced communication with a few different subsystems. At least with regard to fuel consumption and winter driving, that's what the various implementations of traction control and EFI are for. Allows the computer to handle data MUCH faster than a person can, and adapt accordingly.