Still no beta, I suppose they need to get it working on Catalina as it's 64 bit only.
I think what will come common place is multiboot OS as not everyone can get a 64 bit replacement app for what 32 bit ones they have.
That bit of wishful thinking is never going to happen. Apple announced in 2011 that they were moving to 64 bit only—it just took longer than their original prediction of 2015.
I don't know if there's any extra bays in the newer iMacs to add hard drives and how easy they are to get to,
The Keynote was pretty clear on this. PCIe based additional drives won't be cheap but they'll be easy on the new Mac Pro—take a few minutes. There are reasons to stick with external storage as I will explain.
I know the iMac I had, to change the hard drive I had to take the whole thing apart and LCD out to put a bigger drive in it (or change the cpu or clean the cpu fan).
You can partition the drive but separate drives would be better, if you have usb 3.0 you could boot on externals.
So... you don't actually have a Mac anymore?
A lot has gone on with the Mac OS over the last 3 years.
Since High Sierra, do not partition a boot drive for multiple OS. Do not install multiple OS on separate drives if mounted internally. It's a really bad idea. If you've already done this, you should undo it.
Quite ok to install other OS on removable external drives. If, however, you normally boot from an external drive, do not have any OS on the internal drive.
Since OS 10.13 and APFS, the only way to get to the Repair Partition is to boot with the
Command r keys down. If there is more than one copy of the OS installed anywhere in the system, the Repair Partition becomes unavailable. Not an issue if other OS are mounted on removable drives—remove those drives before accessing Repair on the boot drive.
A non APFS volume cannot see or access APFS. Switching from one to the other requires Option boot, no big deal. Only the Sierra and older Repair partitions will be visible, however.
The reason this is important is because of APFS Snapshots. Think of Snapshots as a complete Time Machine Restore in case something goes wrong — except that it takes 1 minute (iMac Pro) to 5 minutes (2010 iMac) to restore a 2TB boot volume instead of 8–10 hours.
I test and download a lot for various reasons. One never knows when you might get into trouble with this. With Snapshots, I'm secure in the absolute knowledge that I can do a complete restore losing less than the last hour of my work — or that I can go back in time further if necessary. It's my choice.
I've tried to find a link that explain this in better detail without trying to sell you some crapware that you don't need. Here's an old article I wrote where I answered questions.
http://www.motunation.com/forum/viewtopic.php?f=4&t=64689