Having played some and looked at some games, here are some interesting game modes I have seen people play:

Only public chat (obviously requires trust) can be used.
Specialist only warfare; all subs going to enemy outposts must contain at least one specialist.
Various specialist restrictive variants, like helmsman/smuggler only
No shields; all shields must be disabled.
Unanimous peace for X days after start
Force gifted specialist; you must gift every specialist you hire to another player, who will gift you a specialist of their own (can be strategic when you gift them a tinkerer to damage their shield, or a king)

Have you guys seen or thought of any interesting variants?

No specialists, No mines, no forces over 60, capped number of generators/factories, and the never played: no drillers. You cannot send drillers, so you must send specialists with 0 drillers and build mines.

You may only send drillers to your East, anywhere between North and South.