Reviews of the Essential phone suggested its main failing was a camera that just wasn't as good as rival phones.
A few years ago Sony made the QX100, a lens and sensor-only camera designed to use an Android phone as screen and storage. Image quality was superb because it was effectively a Sony RX100. However, it was let down by connection and transfer issues. This concept could work well using Motorola's Mod system.
What other mods would I buy, and to what extent can their functionality be incorporated into the base phone? A keyboard - maybe. A game controller - maybe, but Android game support is poor because games are too easily side-loaded. Good stereo microphones for recording jams and gigs - could be incorporated into handset, or otherwise use USB C since separating microphone from phone is useful. A speaker - no, my current phone is loud enough for me to hear podcasts in the shower. Active IR 3D environment mapping - yes please, but I'm weird like that. A projector - nah. IR camera - nah, I don't retrofit insulation to old houses for a living, but it would be fun to pretend to be the Predator for five minutes. Get To Da Choppa! So, there's no 'killer app' for modules for me.