I think migration should not limited to agricultural empires as an activated ability. Migration should be tied to cultural influence. In reality all cities and countries have migration for better opportunities. 

It's very common to have excess population and no open slots for them to work. It's only natural that they would migrate to other cities within the same empire and if no job is available. They would go to another empire where jobs are available. This means that aesthetic civilizations shoud attract workers.

Perhaps there are civics that limit the migration or open them. Maybe migrants cannot be used as military, only home grown population can be. 

For the agricultural ability, this could be changed to a boost of food for a period of turns.

Think of Germany, US or Russia as places where neighboring countries migrate to for a better life.

Any thoughts on this?

I am also in favor of more triggers to migration but I'd not tie it to cultural influence, nor the idea that working opportunities are a strong factor in migration. Migration for better work condition is mostly the apanage of upper classes. Most migrants do so for survival and end up in a neighboring country.

If I like the idea to get a chance to „lose population“ by lack of slots, as if they were jobs, it seems marginal compared to two other triggers which could be:

1. lack of food: any city that lose population due to a lack of food should greatly increase the risk of emigration, it should become exponential with a greater amount of population lost in few turns.
2. proximity to war: bombed cities and or nearby districts should trigger migration easily. Simple proximity to a war should increase it to an extent, even if the player is not directly tied to the war.

I'd see other triggers, less important, being:

1. events, of course (epidemy could play a role)
2. the aforementionned search for a better work place
3. soft power

However, all of that seem to be complicated. The devs would have to add many small functions and make them all relate.
A simplier way could be to trigger migration through stability, factoring nearby cities, eventually trade routes, alliances, etc. Starvation, war, etc. could lead to a loss of stability and trigger migrations. This seems easier to implement and to go well with the already existing mechanics. The only downside is the state of the stability mechanic in the game at the present time.

Concordo que a migração deve ter mais gatilhos, em vez de apenas acontecer ativamente nas nações agrícolas. A migração deve ocorrer de forma passiva, devido a uma grande diferença de FIMS entre territórios, sendo a alimentação a principal, ou devido a guerras. Onde cívicos como tolerância religiosa e multiculturalismo poderiam atrair mais imigrantes ou evitá-los.

Wow, great idea.

I often find myself with a city that exceeds the max population cap,

and I would love the excess population to migrate to another of my cities (admittedly less so to a neighboring empire).

But I could live with it if both options were on the table.