To be fair, the problem would be getting Grodd on Flash season-long. Crossovers I'd imagine being a different story. But the only way I'd see it is if Grodd was part of a Legion of Doom or something. I'd be curious to see Grodd and Lex interact.
He shouldn't be the main villain in a crossover. But if he was with Lex, Bizarro girl, Tobias Whale, and Deathstroke to form some league/ legion. I'm all in.