Generally, increasing the fuse isn't a good idea. Even if there's a seemingly good reason (e.g., a new accessory that requires more current), the intermediate wiring may not be adequately rated to carry the additional current -- think breakdown of the insulation from heat, if not actual fire -- and the voltage drop from the original wire's gauge may make the resulting power to the accessory unsatisfactory/inadequate anyway!

And putting in a resistor in series with your defroster grid is a "fuhgetaboutit" idea! The heat generated by that resistor would probably create a intolerable fire hazard -- really, really hot -- and I doubt you can find a resistor rated high enough (unless you build an elaborate heat sink). The predicted watts generated by this resistor would be R x I^2, and it's that squaring of the current (I), which is not insignificant, that's the killer in this equation! Remember that you would want a resistor who's wattage rating is substantially higher than the wattage predicted!

On the other hand, I have to admit that a slight increase in fuse size might not be very hazardous. Step up one (and only one increment in) fuse size, and see if the problem is gone. If so, perhaps you can get away with it -- what I would do with my own car isn't necessarily what I would recommend to others -- wink, wink ;-). However, I really wouldn't ever increase two fuse sizes -- not even for my own car, let alone a suggestion to anyone else.

Good luck.