While I'm not convinced they "have" to be asymmetrical, I agree that the typical "red is the torso and everyone else is limbs" scenario has been done to death. But I'm not a fan of the Dairanger/Go-Busters (and now Kyoryuger) style "red is the main robot and everyone else just forms armor around him" method either. It feels like making the other mechs less important reflects on their rangers somehow. Maybe that's just me.
To me, the combined form is the icing on the cake. I love to see an interesting looking individual mech, or one that transforms for the combination in an interesting way. I realise that sentai mech design has to take into account both being something that can be a wearable suit and also not make the toy prohibitively expensive, and these both tend to contribute towards a level of blockyness. But whenever they pull off something a little more unusual, I enjoy the mecha so much more. It doesn't have to be something major. The way the dragon origami folds in on itself to go from being a very elongated form to being a compact leg. They way Gosei Snake flips round to be not only the entire of one leg, the waist/groin area, and the upper part of the other leg. Even the way Engine Speedor is just the pectorals and shoulders rather than a full torso. Just mixing it up slightly to make it more unusual. I wish we could see more of that. I'd also like to see the mechs used individually more often. After all, if their only use is to be part of the combined robo, why didn't they just build one big robot rather than having something that seperates into pieces?