Don't you think this is quite a mess? Especially implementation of "common stack" could be problematic...

Certainly not since it is possible already with handwritten code. About implementation the sensitive and insensitive stacks seems to be easier to implement (on error handler add code for a second chance using the insensitive stack). However, with the separated stacks implementation, tricks like the following cannot be done:
Because defining with imacro wouldn't be enough since the user could have defined a conventional macro like nOp and define_all_macros defines "imacro nop" and "macro nop" (the latter as a desperately attempt to prevent calling the user defined macro). But as I said, implementing with one stack is harder and the trivial implementation wastes unnaceptable amounts of memory.

I am not convinced by any of the arguments for imacro (or istruc). No one has shown a case where it is either useful or needed. I think it creates more confusion that anything. Nobody writes mixed case opcodes unless there is a very compelling reason for it (like to deliberately avoid invoking a macro).

But how about imatch? Matching strings by case insensitivity is a useful endeavour IMO. I also showed a use for it in the first post of this thread.

As for code page concerns and the international charset thing, I think that just the ASCII 7-bit codes are sufficient as a starting point. Certainly if it was to be expanded to some form of unicode case match with all the acutes and umlauts etc. then the code becomes quite messy with definitions of code pages and other whatnot.

I write non-capitalized opcodes and so the macros that "overrides" them. Could I share my macros with people who likes to use the first letter capitalized or even with those who use all letters capitalized? (since my redaction sucks I mean things like "Mov" and "MOV" respectively when my macros are "mov")

[edit]Yet, your suggestion is priority one IMHO[/edit]

Does anyone really write their opcodes with the initial letter capitalised and the rest lower case? I have not seen it. And it seems to be quite inconvenient to type like that anyway.

I accept that some people do write in all uppercase, but they are a clear minority. If I ever have occasion to include code from someone that writes in all uppercase I simply use my editor to push it all to lowercase before I do anything else with it.

Can't remember where I've seen things like "Invoke", but yes some (not many) people does it.