What features would you like to add?
For auto accompaniment there are two that I can think of:
1) "one Man Band" I think this is the most like the arraigner keyboards
they have several versions at http://www.1manband.nl/
It can play Yamaha style files
2) "Band in a Box" is better known and has been around "forever" http://www.pgmusic.com/
Both of the above are much more powerful then any of those simple patterns built into the keyboard.
Here is a list of more: http://www.synthzone.com/midiaccomp.htm
And then there is Apple's "main stage" that is targeted at performing professionals http://www.apple.com/logicstudio/mainstage/
but it's main job is sound not sequencing. It can do split keyboard and layering, drums and guitar amp modling and so on.
For any kind of virtual instruments it is easy to find thousands of those includes modeled analog synths
Recording and editing software is easy to find. Two examples are Pro Tools and Logichttp://www.digidesign.com/index.cfm?navid=507&langid=1http://www.apple.com/logicstudio/logicpro/
This is what I can think of and I'm only just getting started. I can think of more titles but I don't know what most of them can do. Pick up the lastest Sweetwater, Sam Ask or Guitar Canter catalog and there are pages and pages of software. I don't know who'd ever have time to evaluate it all