It was once mentioned a new invasion mechanic was thought at, just an idea I was contemplating today:
1. New tech: "
Planetray Mass Invasion" with 3 tiers. Same category as tank battalion and milita upgrades
2. If tech has been researched a new "project" is made available on planets: "Planetary Invasion agains: PlanetX"
3. The option is available for every enemy planet the current empire is blockading. E.g: If player is blockading 2 enemy planets, thre are two such projects listed once for each planet
4. The project cost is 1pop/turn + as many transports occupies from the pool as the travel distance from source to target planet (the futher away the target the more transprot capacity is needed to ferry a constant stream of troops into the invasion)
5. New event message summarising the status of all ongoing invasions at the start of the turn: Planet A has been invaded by 2billions soldiers and has lost 3billion militia
6. A blockaded enemy planet can only be invaded by as many planets as the level of the tech: 1-3 (3 being the maximum amount of planets that can feed troops into the invasion and thus 3b troops per turn max)
7. Invading forces strenght should be x2 that of milita due to their better training and supply (the defenders are blockaded and cut off from supply)
8. If a planet is conquered, the surplus soldiers can either be consumed on placed on the planet and the planet(s) sending the troops get a production finished notification awaiting new orders
9. If an invasion cannot be continued as the conditions for production of the project are not met (insuffiienct pops on planet, transprots in poll or the enemy planet is not blockaded anymore) the planet stops production and generates a message of prodocution canot be continued
The above concept would be combining the current mechanic of planetary refitting ships "projects" and the trade pool
Building up surplus trade fleet pool capacity would be very beneficial beyond the current need for moving around colonists
The option would save a lot of micro around producing and moving invasion forces but would not make tank battlations obsolete as these woud doverall be still cheaper more flexible and faster for conducting lightning invasions or holding a critical planet for a while.
Tanks would be cheaper overall as the amount of pops lost during an invasion would be considerable for an average planet of 10: it should cost around 5pops in average to capture it
But would still be worth in most occasions as the factions dont have dozens of tanks waiting around in transports in most cases and thus the time for such an invasion should be shorter then producing tanks and ferrying them over ad-hoc + no bombing would be necessary to soften up the enemy forces for the tanks to have a chance . As long as there is sufficient manpower on the planets, the invasion can grind down the defenses with minimal infrastructure losses
Tanks and Mass Invasion can also be combined perfectly well as invasion would take the place of the initial softening of resistance via bombing and the tanks can be sent in once the planetary defenses are weakened mimicking the concept of wearing down defenses with low quality troops and sending in the high-value heavy hitters at the right time
The entire concept would also be quite immersive as there would be hard fought planets while the blockading fleet would have a purpose other then just starving out the pops or bombing them out of existence + this would fit in perfectly with a planetary fallout system which is discouraging bombing as it would provide a simplem easy to use and thematic alternative agaisnt enemy planets with high cocnentration of pops ann/or tanks
The concept also reminds be on the siege of Vraks from 40k which is also a plus in my books