Personally, I think the could get away with just making it thinner and faster. What else would a tablet user want?
more stable and if possible powerful processor, better battery life. a nicer camera would be awesome. 802.11 ac wifi. support for h.265 wouldn't hurt either (that would likely be covered by better processor etc). better display quality outside is a must for me. Doesn't have to be perfect but it needs to be better. More sensitive screen for finer writing and drawing wouldn't hurt either
i dont need it to be thinner, in fact I wasn't bugged but the milli thicker change from the ipad 2 to the ipad 3. took me less than an hour to be over that.
On the software front, the only thing out of the various gripes that i consider even close to vital is a way to move common file types from app to app without a computer for those that don't have one. I'm talking PDFs and similar. Or to do things like create a song in Garageband and share it to iTunes to play with your other music, use in iMovie etc. Putting such things in a common bucket by file type would do it. Even if we can't see the bucket directly.
Another thing I would like to see but isn't vital would be a way to 'backup' data separately (ie a file per app not one file with everything) to the computer. Nothing sucks more than having some app thats crashing like mad and being told to wipe everything and set up as new. If things were separate then I could at least restore my texts. I could even possibly restore data from an app I accidentally deleted just by reinstalling it via iTunes and having it ask if I want to restore my previous data (same with iCloud perhaps)
More specific Restrictions wouldn't be bad either. Ability to restrict all apps, even perhaps a way to tag certain bookmarks as safe sites and allow Safari to only access those sites like you can on the computer. Restrict music to a preset playlist and so on
And speaking of music etc. it would be great if they would adopt the same local and cloud in a single list with (when in the area) your home sharing included. Same in iBooks by dumping the separate purchases shelf. And it wouldn't hurt if there was a way to sort by author, keywords etc.
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a. Multi-user account, whereby you can use the account and if you share it with someone, they could always use a guest account, etc.
Aside from perhaps a way to save restrictions settings and create a 'kid mode' that you could move in and out of with a gesture and passcode etc. this is unlikely.
As are the rest. Apple doesn't view these as multi user devices or computers.