If were talking new lenses, with the opportunities the RF mount offers, I'd love to see what an RF version of the EF 28-300MM F3.5-5.6L IS USM would be like?
I tried the EF version and its a beast. It's a fantastic lens that covers so many situations but unless you're a weight lifter, I'm not sure I could manage more than a couple of hours handheld lol. Maybe they could even do an f 2.8 RF version, now that would be impressive for wildlife and travel?
They've released the 24-240 already as a superzoom, I think it's unlikely they'll revisit the 28-300 L line because to be honest, you can't really do a good enough quality superzoom without it being enormously heavy and enormously expensive.