Xeno sounds like a good idea.
You could have engineers.  With engineers you could have Moore powerful lazers,  bomber, or increase the size of your planets or even add a moon.

Oooh!!!  Many good ideas... 

How about instead of many different units, there could be a system whereby a player could choose the percentage of their population to be a certain occupation.  And there would be many occupations to choose from.  For instance, players could choose to have a large portion of their population allocated as 'agrarians', which would increase food production, or, as mentioned, 'engineers' which would provide the above mentioned benefits, or 'mercantilists', which might increasing income bonus, or 'reservists', which would increase military bonus on defence, 'medics', which would specifically increase soldiers' attack power on defence, or 'scientists', which would increase science bonus,  'academics', which would increase both science and magic bonus.  Allocating pop to 'priests' would provide the benefits I mentioned that priests might have except for the ops.

In this case, they wouldn't be new units.  Allocating pop to certain types of occupations would just provide various bonuses, with 'priests' providing the op defence, pop growth, and science bonuses. 

There could be a whole bunch of different occupations to which a player could allocate certain percentages of their empire's population so as to tweak their bonuses, and do so according to their gameplay role / style, and so so over the course of the round, so as to adapt their empire to gameplay circumstances.

We could introduce prostitutes to increase pop growth.

ouch 

or 2 types of expo's which of 1 instantly portals the planet.

Or implosion ships.  When winning a battle the planet would implode removing it from the game completely. Usefull to kill planet ping pong.

My hackers (new unit) would do a ddos attack causing hyperinflation  (new opp).

Lightguns *sneezes*

If the function is implemented whereby pop is actually made to be useful besides generating income for pop bankers, there could be tips such as:

"Allocating some of your population to 'reservists' provides a bonus to your main fleet's soldiers' attack power when defending portals."

"Allocating a portion of your population to priests increases your empire's population growth, as well as increases defence against magic and covert operations."

"Allocating a portion of your population to prostitutes increases the recovery-rate of your empire's morale, but decreases population growth rate."

"Allocating a portion of your population to engineers decreases build-times and costs."

"Allocating a portion of your population to agrarians increases agricultural production."

Additionally, there should be a maximum portion of an empire's population that should be allocated to different occupations, depending on the occupation.  For example, while a maximum of 90 percent of a population could be allocated as agrarians or reservists, only a maximum of 10 percent of a population could be allocated as prostitutes, priests, or scientists, while a maximum of 30 percent of a population could be allocated as engineers, and a maximum of 50 percent of a population could be allocated as miners.

Lastly, if population becomes useful in the above mentioned ways (and others), and if the option is provided such that when capturing a planet an attacker can opt not to kill-off the population, and instead opt to capture the planet's population, and, moreover, if population only earns income and only contributes to an empire's net worth if the population is left 'unallocated' suddenly more options for different styles of gameplay are unleashed and the game becomes more interesting.  For instance, it might become a strategy for attackers to build LQs and house 'reservists'.

That would be a first step towards making pop useful for cool bonuses.