If you're looking for a standard to adhere to, I'd consider implementing the GDB remote protocol. (Link is to Embecosm's guide on writing a GDB Remote Serial Protocol server.)
It needs GDB to support the ISA you're targeting (I've heard it's the same sort of "build a special cross-targeting version" song and dance as with GCC), but, people have gotten GDB to support a lot of architectures (eg. STM8, despite GCC not supporting it) and, in the worst case, you could always implement both sides of the conversation yourself and have a non-GDB debugger that's very friendly to someone coming along later and adding support to GDB because it speaks a standard language.
Here are some things which implement it:
- gdbserver is GDB's own tool for letting a GDB instance on one machine that can run GDB debug a program running on another machine that can run GDB. (eg. So you can debug something on a co-located server using your local copy of GDB. It's also what Android uses.)
- Wine's WineDbg has a GDB mode.
- The KDB/KGDB debugger supports GDB remoting over serial. (According to Wikipedia, it's available for Linux, FreeBSD, and NetBSD kernels.)
- QEMU (x86, x86-64 PCs, MIPS64, SPARC, ARM, SH4, PPC, RISC-V, etc.) has an implementation. (Here's Xilinx's guide for using it to debug ARM stuff and here's a post on how to use it to debug real-mode x86, because the QEMU wiki page I linked is such a stub.)
- The Bochs x86 emulator can be built with GDB support.
- VMWare Workstation (but not VMWare Player) implements GDB support.
- Someone's written a v0.1 pre-release of GDB support for VirtualBox.
- Apple's debugserver for iOS implements gdb protocol as does Google Project Zero's ktrr for kernel-debugging jailbroken iPhones.
- OpenOCD provides a server to translate between GDB remote protocol and the on-chip debugging functionality of programmers like the ST-Link. (The programmer you can get a $2 Chinese clone of to program your $1 Chinese STM8 and $2 Chinese STM32 dev boards.)
- Various official devkits include GDB remote support, such as the official STMicro ST-LINK server that OpenOCD serves as a less proprietary alternative to.
- The BlackMagic probe is firmware you can flash to an ST-Link or buy flashed into purpose-built hardware which implements programming and the GDB protocol directly on the device for various ARM Cortex-M and Cortext-A devices such as various STM32 sub-series.
- According to the Qt Creator Connecting Bare Metal Devices page, EBLink and J-Link also implement GDB protocol support.
- Though they don't mention it on their site as far as I can tell, mGBA has a GDB implementation that can be enabled from the GUI.
- According to this 2020 StackOverflow answer, VisualBoyAdvance-M has GDB support, but this post said that "VBA gdb connection hasn't worked in a very long time" in 2018, so I'm assuming that VBA-M un-broke GDB support added by the original project.
- There are patches for certain revisions of DOSBox.
(It's sort of amazing that some things running compatible ISAs don't implement GDB support natively. VirtualBox's built-in debugger is described as being like the OS/2 and CodeView debuggers.)
Here are some GDB frontends which explicitly list support for remote debugging:
IDA Pro also implements support for debugging things that speak GDB's protocol if that's your preference but x64dbg does not  .