I'm not sure about the plot of Diablo Immortal. On one hand, it shows a lot of respect for the history of the series it picks up right where Diablo II left off and setting the scene for the events to come to follow in Diablo III. It will also provide more information about the cities and civilizations of Sanctuary and fans who have played for a long time will appreciate appearances from characters such as D2R Items the Necromancer and King Leoric from Tristram.
However the narrative is almost a beat-for-beat recreation from Diablo II. You'll assist a group of woodland rogues, reassemble legendary troops at the edge of the desert fight hostile Shamans in a swamp, or climb an mountain with barbarians- if the game is Diablo II, you've literally experienced all of these before. The story beats in the new game come down to "find the MacGuffin eliminate the MacGuffin" repeated multiple times. I also wonder whether Blizzard will utilize this formula for adding new areas to the game in the near future.
The way Diablo Immortal plays and feels depends on whether you're playing the full-release version for Android or iOS or the beta version on your PC. (Your save data syncs across the two, so it's not necessary to commit either one or the other.)
On mobile, it has impressive graphics and impressive performance. You'll need to have a fairly powerful device to run it well. On my Pixel 4a, the game lagged and chugged often, and the machine ran extremely hot. It's also very difficult to keep an eye on all the action with such a small display in particular when you have to focus on specific abilities. You can make it easier by playing on a tablet and using an buy d2r items appropriate controller. However, then you can simply play on the PC.