I have done my share of instructing and have a few thoughts

Must be very hard to stall, OK hammerheads are not preventable but a good trainer can be dragged of the ground semi prop hanging then, horsed around in tight slow turns with mammoth control movements without stalling.

+ 1 on the dual leg nose gear. But single leg is OK as long as the leg can be easily replaced at the field.

Must present a good recognizable silhouette in the air so the beginner does not lose orientation. Training on a delta or a Zagi is a bad idea.

Spare parts should be available eg you can buy a new fuz if the wing survives as they often do.

When I learned to fly back in the 60s a good trainer could recover itself from any situation and resume normal flight without losing to much altitude if you just put the tranny down and walked away. I am not sure that this is still needed with an instructor and buddy box. I have no experience with the so called safe autopilots but can see the value to someone trying to learn without an instructor.

Despite my love of IC engines maybe electric is the way to go.

Wings should be attached with rubber bands and not bolted on.

Trainers should have a light wing loading and be a little overpowered so they can power their way out of trouble without the learner having to baby them back up to flying speed.

BIG IS BETTER. The best trainer I ever used was a 12 foot span Telemaster. Things happened SLOWLY.