I've seen some heavily converted armies in the past that used both Skaven and Lizardmen and they were pretty cool.

A player made Space Rats by combining Space Marines and Skaven models. They basically had marine/space wolf bodies with Skaven heads and tails popping out of the armor. He had a lot of dreadnaughts that had clockwork gears and painted the armor to look like wood. I think they maybe had a lot of lightning claws IIRC.

Another player made an army out of skinks and saurus models. I think he ran them as either orks or IG, but he had a lot of heavily converted models that he used as tanks/trucks. There were a lot of Catachan arm swaps so things could hold guns.