Ideally it should be neither iOS or Android, but a free software OS not focused around ads. Parts should be modular and user-replaceable, like the Fairphone or the Framework laptop. It would cut down on e-waste caused by either damage or routine replacement. With modular components, you could upgrade the mainboard, storage, or camera after a few years without having to replace the whole phone. It's already possible with today's technology, just not good for profits.