Of the bats we've seen in Pokemon, we have one line that is primarily based on vampire bats whose entire diet is blood, but have sometimes been depicted in fanworks as sometimes eating Bug-types similar to most bats in the real world. Then we have two lines based on fruit-eating bats, one of which begins as a Honduran white bat and evolves into a flying fox, and one line that appears to be flying foxes all the way through (and are currently the only mammalian Dragon-types, a Dragon Eeveelution would be the third).
We haven't seen a line based completely on insectivorous bats. Since the Galar region is based on the UK, I'd like to see a line based on common pipistrelles, with an initial stage based on pups that can only be obtained by breeding, but ultimately it is a three-stage line. It would be a primarily Fire-type line (based on Fire being strong against Bug, a similar thing is seen with the anteater Pokemon Heatmor), and gain the Flying-type in its second stage, the stage most common in the wild (the third stage would be mainly obtained by evolving the second stage though it can occasionally be caught in the wild).