I love fish shell, I love the autocomplete, but I've been forced to switch back to bash lately, because fish syntax is just different enough from bash-isms to break too much scripts / tools / commands.
I know I could just invoke bash, but prefixing commands with "bash -c '" is tiring and papercuts are why I switched to fish in the first place. I wish bash had a sane auto-complete prediction like fish.
I've had success with bass[1]. Typically, I use it when some kind of tool requires sourcing bash scripts.