Software Package Data Exchange provides standard codes for lots of licenses. Why not to use them in the new Module:Extension?
Topic on Template talk:ExtensionLicense/Flow
SPDX is already recommended in Manual:Coding_conventions#Licenses, so you can certainly go ahead and implement it. However, the issue is with hundreds of extension pages using unrecognised values: are you able to fix those as well?
I went bold and implemented my proposal, then requested the bot flag to fix extension pages.
They have WTFPL (version 2, adding 1 was rejected as not relevant), not bad. Sadly not directly helpful for commons (no FAL), and they're still trying to figure out Unicode licenses. Nice list, use it.