Realistic alt modes.

This includes:
+ Devices/objects, e.g. toaster, phone, camera, walkman, cassettes, stereo etc.
+ Licensed vehicles
+ Realistic looking beast modes

Rationale: Robots In Disguise!
Spaceships and future cars don't look realistic (some of them don't even look aerodynamic), and robotic animals and insects aren't fooling anyone. Although I did like how the G1 comics and ROTF had insect-sized Insecticons (G1 comic Insecticons could mass shift from giant robots to insect-sized Insecticon modes). At that small size (and often buzzing speed), it's less likely that most people would notice that the insect is mechanical rather than organic. Some of my favourite moments have been seeing Transformers infiltrate human society with their disguises.

I really love it when I see a vehicle, device or animal IRL that's the same as a Transformer's alt mode, as my mind just immediately visualises it transforming. e.g. every time I see a Champion Yellow Suzuki Swift, I imagine it transforming into Alternity Bumblebee (heck, if it's a blue Swift, I imagine it transforming into Alternity Glyph, if it's white, then Alternity Bug Bite and so on) etc.