I propose another Addon-App (similar to termux:boot) that gets invoked by termux:command URIs.
Doing this in a seperate app would outsource all the security issues from the core app, be easy to replace and so on.

Control Flow:

1. User somehow opens a termux: URI
2. Termux:URI parses the URI into:
   • Command and args
   • Foreground/Background
   • possibly more (CWD, ENV, ...)
3. Termux:URI does a security check
4. Termux:URI sends the command to Termux

URI Encoding

Possibilities of Encoding of command and arguments:

21. leave as is: termux:command arg1 "arg 2"
    might not be supported by some apps
22. URL Encode termux:command%20arg1%20%22arg2%22
    looks ugly
23. base64 Encode everything termux:Y29tbWFuZCBhcmcxICJhcmcgMiI=
    looks dangerous
24. let the user decide (see 35.)

Possibilities of Encoding of Foreground Background:

25. if the last char of the command is & its Background
    feels very unix
26. different URI Scheme: termuxbg background.termux.com background.v1.uri.termux.com
27. URL Param termux:command?background=1
    would work well with cwd, ...
28. Configurable outside of URI (see 35.)

Security

My Ideas for security:

31. Ask the user: "Dou you want to execute command arg1 "arg 2"" [YES] [NO]
32. Configurable Whitelist, Add Button [YES and don't ask again]
33. Only commands in .termux/uri/ can be executed.
34. A whitelist in textform in .termux/uri/whitelist with one line per allowed command
    • do full match ?
    • do prefix match? must clean chars like | ; && could be unsafe with some commands
35. call one single script, .termux/uri-handler with the complete uri, skipping Step 2 and 3 and let it decide what to do.

Additional features

• Depending on how complicate Step 2 is, a wizard to generate the termux uri would be neat. either in the App, or in javascript, hosted on the termux website. (producing a qr code)

• Manage the whitelist

Benefit

start commands in TERMUX from:

• all kinds of automation Apps
• QR Codes
• NFC Tags
• other shiny new things

If I'm not mistaken, this would (help to) fix these issues:
termux-tasker # 6
#547
#804
#924

And now?

I have no clue about android programming, but I might enjoy playing around with it. I guess starting off of the termux:boot app to get something working, but I would need help making it into proper android app that follows some kind of coding standard etc.