Warbler said:

even without the "To Be continued...", It still seems ripe for a sequel to me. The movie ended with them going into the future and the Doc saying "somethings got to be done about your kids!" The ending never says what needs to be done about the kids, it never says they succeeded in doing whatever needed to be done about the kids. It just ends with them time traveling into the future. Sure seems like a setup for a sequel to me.

The ending was meant as a joke, not to establish the possibility of a sequel. IIRC this is confirmed by Bob Gale and Niel Canton on the 2nd audio commentary track on my 25th anniversary Region 1 DVD. The pair explain that if a sequel had been in mind during the production of the first film, Claudia Wells would not have been in the Delorean at the end with Fox and Lloyd and this is why Elizabeth Shue spends most of Part II and III "asleep."