I'm really looking forward to Bootstrap 4, it's been one of those key productivity boosters in my arsenal of skills. So, many thanks to the Bootstrap team, you've made the world a better place (if anything, certainly my world).
Though, one thing I noticed going through the docs, the first thing I always look at is the "forms" section. For most apps, forms are probably the most important aspect (at least, for me they are).
I noticed that there are still two type of forms, normal ones and inline. However, when I work on larger apps that require more sophisticated input I often require a mix of both normal and inline forms. Maybe when you're working on SPA's it's easier to have multiple forms on a page and treat them as a single form but for old-school traditional page submit apps it's a little bit more difficult.
Anyhow, I wished they'd make the "form-inline" style more a form group thing rather than a form element thing. Currently the forms have a "form-group" which only contains a single control. To me (and correct me if I'm wrong), "form-group" should actually be named something like "control-group" so that "form-group" could be used to have both a normal and inline style form within the same form, maybe the ability to apply "form-inline" to a or something. If that makes sense..
Anyhow, I'm rambling. Love Bootstrap. Keep it up!