OK I just read this again, and as a mobile system analyst of one of the biggest site in Canada, i'll try to pitch in
1. As previously mentioned, having live streaming data to the app is a big NO-NO. This means developing a specific API that a mobile device can access at any time. This also means impacting the megaserver client connections which, most likely, would be detrimental. Best way to go around this is to develop a separate gateway for these data which presumably will be refreshed periodically. Like every 24 hr, or 12 hr or something. This means a lot more resources to work on.
2. Some feature requested like reading lore-books and notes, are HUGE data. with mobile, we don't want people to download a 1GB app. Space is a luxury. This means big datas like Lore-Books would be cached in the server, and only ones the account has discovered is downloaded to the device. Then see point #1 above again.
3. Gated by a separate server means stuff that can pro-actively sync back to the server becomes complicated. Stuff like changing skill bar, or feeding your mount, etc... Meaning, the stuff that goes back and "write" back to ZOS, not just stuff the app "reads" from ZOS. I personally prefer having lag improved than adding stuff like this, back to point #1.
4. Chat is the only exception feature that I can see (and must anyway) to be implemented directly to the megaservers. A bunch of text is significantly a lot more compact than skill bar information.... I can totally see this happening with no detrimental effect to server.
5. I'm personally against this, but with enough feature, I can see how this warrants TESO+ membership only. It is a possible revenue source and I'd rather have this than having ADS inside the app...................
6. Crown store is definitely do-able as it basically a separate instance and a "read-only" function, where purchases is not required to have a live connection to the megaserver.
7. Crafting is a big hurdle too because of one single component: the by-chance logic. Some function in crafting consist of percentage chances.... this means, whatever the result of the "lottery" is calculated and recorded on the server side... meaning it's most likely need to be connected to the megaserver too...
That's all I can think of for now. Sorry to rain on your parade, but in summary, unless ZOS put some serious resources to develop both the app and the infrastructure, they're not going to be able to pull it off WITHOUT affecting the lag issue.......
(or they can go cheapskate and just simply build an app to plugs directly to the megaserver and ignores the tear of the players with +999 ping....)
Having the ability to mess with inventory on the go would be AMAZING!
Does this fall too much into "gameplay" @newtinmpls ?