What will be options and advantages of espionage? List benefits with each level of espionage if you have some ideas, and also sabotage options. I also don't know how may 'levels' of infiltration there will be.
Espionage:
low levels: View every pact and treaty with other empires, view balance budget and intenal politics, view basic info (how many ships they have, how many planets, no. of techs researched, trade routes etc.)
Mid levels: See all their technologies, gain vision on all of their trade routes, can see exactly what ships do they have and how many of each, more information (currently researching tech, best trade route, best planet etc.)
High levels: Steals their world map, can see exact location of their ships, can see basic info for every planet

Sabotage options:
Steal technology- no explanation needed, except that you could also steal tech that you don't have prerequisites for, but then you gain buleprint and as soon as you research prerequisites for that tech, you gain it. Or maybe you still have to research it, but at high speed.

Deploy spy sattelite (Requires spy satelite tech) Gain info on specific planet: Population, social improvements, buildings, etc. Remains deployed for some time. High chance of succes.

Sabotage planet- destroyes currently building planetary improvement or one you choose.

Terrorist strike- reduces morale, kills lots of people and destroyes some infrastrusture. Nasty if you get caught.

Plant disease- Very contagious virus epidemics. Not lethal, but lowers productivity. Since it's difficult to infect entire planet, effect should be lowering production by 30-75%, and this varies every month (Spreading or fading of disease). Also, epidemic should last for some time, and if planet has trade route, disease could spread on other planets.

Propaganda- Lowers happines and increases chances that planet will defect to you. You are always detected, of course

Foster war- Scenes incident which provokes war with 3rd party. But most of the times it won't end in war, of course, but it may result in breaking alliance, ending trading partnership, imposing sanctions or embargo, and all-over decrease of relationship. Bad if you get detected, because you get nailed by BOTH sides.

Disable defences- Disables planetary defences on selected planet. No need to say how useful if you are invading that planet.

Send pirate attack- Very expensive, but enemy get tons of ships harrasing his economy, disrupting trade routes and looting undefended colonies. Easy and almost risk-free, as pirates rarely talk.

Maybe you should be able to negotiate with pirates other than this way. Maybe to have them as minor nation with specific choices, like hire merchenaries, pay tribute for leaving you alone, send them to opponent as mentioned above, even selling contraband to your planets and some black-market trade with civs on which you have embargo...

You should also be able to get some events if you have high espionage level, for example "Our spyes have found security failure in Human ships, and group of their ships located at /coordinates/ are now easy target for our spec-ops" You can use this special opportunity and steal ships, although this reduces relations and costs some money, or pass it.