I agree, other than adventures? We'll see how easy it is to change monsters....
The monsters will be compatible. I'm building it based on the same math, so monsters will have an XP value that allows you to use them in vanilla 5e or 5.5. I've used the model for both 5e and Odyssey (my 5e-derived engine), and it's working well so far.
The character options are built to allow for faster play without (hopefully!) making them too simple. Ideally, players would feel they have 5e-level characters in terms of customization and unique abilities, but OSE or Shadowdark feel in terms of speed at the table. I'm working on a game design document to share on the Patreon early next week.
One note about the Odyssey engine - it drops the damage bonus to attacks. That might seem small, but especially at low levels it opens up the power budget for class abilities. For instance, an Odyssey fighter gets two attacks starting at level 3. I think that class features will end up feeling more narrow but impactful.
Multiclassing is still down the line, but my idea system would look like the class creation system from the 2e DMG. That system assigned a point cost to each class feature, and you built your custom class with a budget.
Most of my games for the past 5 years have been online sessions played one hour at a time. I'm using that experience to drive the design. One hour, online games have made ongoing campaigns much easier to keep going for me. I have three active campaigns despite working full-time, raising a kid, helping take care of elderly relatives, and doing all the other stuff that comes with home and family.
Honestly, I was surprised that removing bonus actions kicked up that much discussion. If I did, I would've dropped a more direct link to my Patreon in the thread!