They have to decide only about one thing whether to go D2 or D3 way - drop chances. Everything else including itemization could have different mode representations.
The reason different modes for drop chances won’t succeed is that every player will jump on the higher drop chances mode.
Imagine that right now in D3 you have a choice to farm with the usual rate or farm x20 the PTR rate. 99% of the player base will jump on the x20 rate (and even if you enjoy the slower rate more, eventually you will jump on the faster one simply because there won’t be any other players on the server except you). Same would be the case for paragon if it could be earned faster - people simply WANT IT ALL. That is in our nature.
PoE, D2 or D3 drop chances then?
It really depends on how they structure the game, namely the Seasonal experience. This depends on the frequency of the updates we are going to receive, which depends on the popularity of the game and the player pool.
Best case scenario is huge popularity and massive player base. This means very frequent updates and Seasons refreshing each month with new content. In such case D3 drop chances would be optimal. Seasons lasting 3 months would benefit more from D2 drop chances, while a NS scenario would benefit most from PoE drop chances.
Another option is they structure the experience not around Seasons rather around Leagues, which would be customized Seasons. Say you have an option to choose how long your Season to last from 1, 2 or 3 months and depending on your choice you will be matched with the respected player pool.
They have options how to approach the Seasonal experience, but for the drop chances they don’t.
Also, power creep is another factor that can turn players off and the more frequent the updates are the higher the chance power creep is introduced with each of these. So, the developers have to balance between content updates and power creep smartly.
An extreme option would be character aging to be introduced and NS mode completely removed. This would mean the characters will die at the end of each Season and the farming for gear and stats would have to start a new with each and every Season (just as in real life - you are born, you learn things and eventually you die). This then would make power creep irrelevant.
Many options for many things except the drop chances. This one simply has to be decided and not touched further since this would create power creep too.