The thing with those "curveball" castings is that the reasoning behind them always came with a sense of wanting to reinvent the entire character. It made sense for Raimi and Eddie Brock, since Raimi didn't even like Venom in the first place. And it also made sense for Snyder and Luthor since he wanted all the real-world parallels to Zuckerberg or whatever and since his entire philosophy of BvS and MoS was to make it reaaally contemporary it was actually a natural direction for him to go.
There is no real reason to believe that'd make sense with Gunn and Batman though.
In interviews even as far back as the TSS press tour he referred to him along with Superman and Harley Quinn as characters whose characterization is already so well-written and defined in the comics he only needs to stick to that and remain as faithful to that as possible. The inspiration he already singled out for this movie points to a very particular type of Batman, the comics he likes do so as well, and so on.
As far as Hugh Jackman goes... well, he was a head-scratcher because he was a complete unknown. It's extremely unlikely a complete unknown will get cast as Batman for this because while for young actors (as Hugh was at the time) the prospect of them getting their big break as the lead of a blockbuster is not really that far-fetched, it does become waaay more unlikely for actors in their 30s-40s since if they've been in the industry for that long and haven't been lucky with anything, it's very unlikely they'll suddenly get a role as big as Batman. In fact I can't think of any actor in that age range as the lead of a superhero movie that you could argue was a complete unknown like Jackman before they got cast.
Heath is a whole other topic since in his case you're talking about an objectively brilliant actor and a brilliant pick for Joker that mostly got **** on because of poorly veiled homophobia and hatred of teen romance movies. I can't think of any actor for Batman that'd generate that sort of backlash while also simultaneously being an objectively perfect pick for the part. The one actor of that ilk was Robert Pattinson, but I don't really think there are any equivalents for DCU Batman.