Nobody was thinking sequel when the original film wrapped.
they weren't? did you see the same ending to first movie that I did??
They actually weren't, the movie was supposed to be a self contained story. If you're referring to the "To be continued..." title card, that was added in the vhs release, it wasn't in the theatrical release.
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.