A Bluetooth keyboard is possible, so a Bluetooth mouse/trackpad is at least theoretically possible - USB need not apply. When I'm writing (professionally) on my iPad, I've often wished I had a mouse to supplement the Bluetooth keyboard - reaching up to the propped-up iPad screen is not optimal as far as I'm concerned.
Would Apple add iOS support for pointing devices? I wouldn't put money down on it, one way or the other. Siri support for pointing/selecting seems a safer bet, as it would be an accessibility feature as well.
Now, is a higher pixel density a good reason to favor a tiny screen over a substantially larger screen? Not in my book. Talk about bad ergonomics! Eye strain is a much bigger issue for me than arm- or neck-muscle strain.