I'd like to run Termux as root: possible?
The default su from Termux is simple. When run it brings /system/bin/sh for you instead of Termux's shell (typically bash). To solve this issue, you can install the custom su wrapper for Termux by  

apt install tsu

And then you can run tsu so that your bash and everything is now under root privilege.  

Further information:
Use an editor (like vim) to see /data/data/com.termux/files/usr/bin/su and you'll find that at line 10, it calls exec su $@ directly while in tsu, it performs shell detection and at line 80, it runs  

exec su --preserve-environment -c "LD_LIBRARY_PATH=$PREFIX/lib $ROOT_SHELL"

to preserve the Termux enviroment.