ZBX-2866 should likely be implemented first 
The risk here is obviously with user macros that resolve to themselves. Note that such recursion might not be direct. Another difficulty could be with the implicit unquoting.
I think that you can use zabbix macros as value of user macro.
I think in this case, you can prevent loop in recursion.
What do you think about?
I tried the same today, with the "Template Virt VMware" where you have to define "{$URL}" on host level to resolve to "https://1.2.3.4/sdk". Now I thought I'm smart like @eht16 and resolve "{$URL}" to "https://{HOST.IP}/sdk" on template level. But sadly that won't resolve. Are there any plans to change the behavior of Zabbix in this way? Until then I have to set "{$URL}" on each and every host manually :-/
I want to use "{$URL}" = "https://{HOST.IP}/SDK" for my VMware ESX Template.
Zabbix should include nested macro support. This is very flexible and wanted functionality.
Grafana supports nested macros. This is a common thing for templating. And if Zabbix is based on templates, then should not cut down all the flexibility, if properly made recursion checks.
This feature would be really useful for me as well.
It would be even better if we could pass LLD macro behind a user macro, for a greater host level LLD flexibility.
My use case is:
{$SPECIAL.1} = asdf
{$SPECIAL.2} = qwerty
{$REGEX_ALL} = {$SPECIAL.1}|{$SPECIAL.2}
{$REGEX_ALL} would be used within a trigger expression in a trigger prototype.
When a need for {$SPECIAL.3} comes along, I could simply add it to the template and update the {$REGEX_ALL} macro.
Currently, I have to edit the trigger prototype, and then force a discovery on every host if I want the new macro to be seen immediately.
This feature would be really useful for me as well.