Off the top of my head, take your pick of what could be improved in any phone that ever existed:
Better camera(s), brighter/faster screens, more durability, more repairability, faster CPU, more storage, more RAM, more battery life, a SD slot, a headphone jack, thinner phone, smaller phone options, more connectivity options, better sound, better waterproofing, better security/privacy...

Want even more innovation that would require hardware? Ok: thermal imager, thermometer (ambient / medical), physical lenses or filters for various light situations (low-light, high-light)...

Now whether any of those innovations make any commercial sense at all is a different discussion, but let's also not pretend there cannot be innovation or improvements to current phone hardware...

Better camera(s), brighter/faster screens, faster CPU, more storage, more connectivity options

You are getting all of that in this phone.

more battery life, thinner phone

Those two are incompatible.

You aren't going to get a headphone jack on flagships.

thermometer

They had that, and people made fun of them for adding it, and people weren't using it.