What do you think Jace dreams about? Not as in goals, but literally dreams while he's asleep? I'm sure it's changed throughout his life.
Oh man this is a fun question... I've done kind of a lot of thinking on this subject to be perfectly honest so hopefully I can get the thoughts together in a way that makes sense.
First up: the "lost memory" dreams. Basically, just that kind of dream that doesn't make a lot of sense but just sort of is a lot of sensation and disconnected ideas/thoughts. In Jace's case, made of up the remaining fragments of his lost memories, kind of all jumbled together into a blurry mess. Probably doesn't have these anymore since the Flood, though.
Next: Nightmares. He probably has these a lot, either replaying memories (being tortured by Tezzeret comes to mind) or fabricated ones (possibly his mind coming up with "bad futures" such as him and his friends dying to the Eldrazi and so on). Poor guy has been through some shit, so PTSD derived nightmares are likely extremely common.
Third: bizarre symbolism. These are my favourite, I would need to write them down in a coherent way but I've speculated at people about intensely symbolic dreams about things, especially his connection to Ravnica as the Guildpact. Stuff like the Soul of the plane speaking to him, or visions the plane herself gives him, though they're weird and don't make too much logical sense.
Last: dreams made up of things he's "experienced" in others' minds, and dream projecting. Just regular dreama but cobbled from the thoughts of others instead of his own. Probably more common when he was more direct about reading minds, but constant. Semi related, I bet he can "project" his dreams unconsciously, sharing them with other sleepers in the vicinity if they're particularly intense or resonant. Probably weirds out people like Ral, Vraska, or Kallist, though someone like Chandra would be less bothered, imo.