I think it'd be cool if you could get a card next round by choosing to get fewer armies to place this round. I primarily thought about Spy, Reconnaissance, and Surveillance cards but maybe other cards work too. I think it'd add a bit of new strategy, as you can decide whether you want to prioritize expanding, or use cards to gain some advantage, like finding out where your opponent is.

This can be implemented in many ways, for example by having a limited number of available cards per team, or both teams compete for a limited supply of shared cards.

+1. Tho it should be a setting to enable or disable

This would be a cool mod for you to make UFO :)