While the rumors swirling around him might be a dent, as far as we know Zenos was never formally disowned by Varis, so he would be the legitimate heir to the Garlean throne. He said he wasn't interested in assuming it, but if it gets him a fight with us quicker I'd see Zenos taking it up to draw us out. And while modern folk might not be terribly interested in following a bloodthirsty tyrant, Garlemald has quite a lot of warrior culture, so while the Populares will likely oppose him the Optimates probably would not...
I mean, the man beat down an Ascian in his own body, using a borrowed corpse so effectively said Ascian (a nigh-immortal, immensely powerful sorcerer) decided to get the heck out of dodge. He singlehandedly turned the tide of a war. If anybody opposes him Zenos could probably kill them pretty easily; the number of people able to go toe-to-toe with him and live to tell the tale can be counted on one hand.