small ditch in living concrete

1949 house is on a slab. remodeling my living room, ripped out the hardwood floor and subfloor and discovered a small ditch in the middle of the it. The ditch is about 4" wide and about 12' long. It was for burying a water pipe which is no longer in used. It's filled in with mostly loose sand/dirt. I'm thinking to make a depth of about 3 1/2 inch and pour cement. Should I bother with it or just leave it? use just regular portland cement or something stronger like CementAll?

Also, there's a thin(1/8") and long crack on the floor, what should I use to fix this?

I'll be installing tiles on top.

Attached are some photos of the ditch and crack. Thanks.