The simple answer? A bug free Mach3 with the Tempest planner.
Seriously. What features are lacking from Mach3? (If everything worked correctly)
The vast majority of users that are moving on from Mach3 are doing so because of bugs that they can't workaround.
OK, let's see.
I think the ability to customize screens is very important. Unfortunately, the people writing control software have show that they are not very good at GUI design.
And no matter how well thought out the GUI is, there;s a good chance that someone else can improve on it.
G41/G42 - To me, this is mandatory. Anyone that's ever run a "big iron" router knows that using G41/G42 is very common. In our shop, I have at least $500-$1000 in 1/2" router bits. After sharpening, none of them are 1/2" anymore. We often run hundreds of unique programs every day, and being able to program for a 1/2" bit for all of them saves an incredible amount of time.
What if I program for a .489" bit one day, and it get's dull before I can run my part? The new bit is .474". without G41/G42, I have to go back and redo all my programs. With G41/G42, just change the diameter in the tool table and start cutting.
Having worked this way fro 20 years, why would I want to go backwards
G43 - Imo, not needed unless you have an ATC, and even then you can work around it. Thousands of people are letting Mach3 automatically zero tools during toolchanges with my screenset and other macros with similar functionality. G43 might be a little faster with a lot of tool changes, but requires a method to have fixed length tooling, which most routers do not have.
Macro or scripting support - I could live without this if the features I'd use scripting for were native functions.
What I need is support for two independent Z axis, via a SwapAxis() type function
Auto Zero during tool changes.
Customizable homing.
User definable park positions.
If all these were native functions, then I wouldn't need scripting.
I don't use offsets very often but they are needed in some situations.
Actually, a bug free Mach3 with Tempest would be all that I'd really need.