Overall, I like your system, I'm always a fan of consistency, however I do have some thoughts & suggestions. Please make of them as you will.
1. Though I agree that using a hash prefix is not necessary for immediate operands in your syntax, I think using it makes it easier for a programmer to tell, at a glance, if something is supposed to be a literal number or a label, particularly when such is indirected through an equate, so the number does not actually appear in the instruction's source. Since I can't see it doing any harm either way, you could make it optional; would also make it slightly easier to transform source code from the traditional syntax to your new one.
2. Whilst I have no problem with keeping the existing Branch instruction mnemonics, perhaps it might be possible to have an additional syntax along the lines of
Code:
B C,label ; instead of BCS
B !C,label ; instead of BCC
B label ; instead of BRA or BRL
B V,label ; instead of BVS
etc, etc.
I think this syntax more closely fits with the rest of your mnemonics, where the operation is kept seperate from the operands. I've always felt that the operation of the Bxx instructions is simply "Relative Branch": the nature of any condition or length of branch is an operand of that branch, but the fundamental operation is still the same. I also, personally, find the non-straightfoward mnemonics unhelpful (e.g. what do BPL, BMI, BNE and BEQ actually mean in terms of flags? I have terrible trouble remembering and normally have to look it up in the programming manual).
3. Similarly, I would suggest XCE -> SCE, in order to match with XBA -> SH, just to keep the "Swap" nomenclature consistent.
4. As the WDM instruction is explicitly stated to be intended for future expansion (even if that's increasingly unlikely to ever happen), I suggest giving WDM an explicitly required signature byte (which could default to a NOP if not specified, if you like), whilst NOPL assembles as a 1-byte instruction for skipping the following, or even rename the latter SKIP to make it even clearer exactly what its for. That way, I feel, the instruction called WDM more closely resembles the intended purpose of the instruction of the same name in the traditional syntax.
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