I can completely empathize with your wanting to make
IG look more sleek and modern. One route you may consider going is combining
IG infantry and another imperial faction as allies for vehicles. I personally considered getting a force of imperial knights painted in
IG gree supported by a platoon of basic troops. This way you can skip the WW1 looking tanks and go for a "walker sci-fi" aesthetic. People have done wonders with that:
I too don't think it's worth converting Tau vehicles, as the dimensions are otally wrong and many people would not play against them if they are meant to represent a Leman Russ tank. (Mostly due to issues of cover etc) Now if you are looking to do a purely artistic conversion I don't see why you couldn't.
The other option is you can create a human manned Tau army if you like the hover technology. Read up on Gue'vesa (human auxiliaries in Tau armies)
It might be a fun conversion project but it would require a bit more labor to put Tau guns in the hands of humans.