With the exception of theocracy, it can be viewed in a fairly linear fashion. Democracy-Republic-Monarchy-Tyranny. I'd say just put Theocracy in the same slot as monarchy and give the ruler the choice there.
More or less. Game-wise, there's no longer any difference between democracies and republics. They may as well be replaced with an "Electoral" government system.
I'm also unsure if tyrannies give the ruler bonus theocracies and monarchies do... If they do, both tyrannies and theocracies would then appear to basically be the same as well.
