My bet will be initially a hybrid design, with all engines operating for high drag regimes and in flight shutdown once at cruise condition. We may see a single donk providing generator output for 2 electric propulsion units. This design would lend itself to a return to 3 engines or twin electric, single turbo propulsion, which could also provide pressurisation and heating via heat recovery. The benefits are a much higher propulsive efficiency over jet thrust, operation at design point, heat recovery put to purpose for what would be otherwise wasted energy plus a greater contingency. You could even shut down the propulsion unit and run off of batteries whilst holding! The dawn of geared turbofans lends itself towards this, albeit with much higher gear ratios. The important tech is the ability to handle the enormous torque required to be used to generate electricity for 2 electric propulsion units.
I think all electric is a pipe dream. Too much power required for the likes of pressurisation, heating and ovens to go all electric. We are struggling to get past 100WHr/KG with sulphur battery technology at present so I think hybrid has to be the most likely commercially usable solution. We will still get a significant reduction in CO2 usage.