I'll declare my interest; I like the 150/152 as a trainer.
Ab initio training doesn't need Cirrus style glass cockpits. Yes a Garmin/iPad nav box for xcountry exercises, but students can move up once they've mastered controlling the 'beast' in the circuit.
Many replacement a/c have missed an important point. Students want to climb to height asap so they can learn upper air work so low power cheap running cost machines actually cost the student more in lost performance.
My plan would be to reduce weight by replacing as much material with modern strong light composites as possible.
Next find a power plant that can run on fuel cheaper than AVGAS and match or better 115 hp. Think about a wobbly (electric) prop to help takeoff and climb performance.
And finally, only employ waif like instructors.
Oh if only I could afford to put my idea into practice.
SGC