Interesting prototype! Things I would be interested to see in further revisions/versions:
- differences in the element tracks (frequency of upgrades, ways in which they give you points, etc). Mainly increasing the frequency of upgrades for the culture track, given the fact that it doesn't multiply at the end.
- double sided cards with elements on both sides (that could be done by either rotating the cards 90° when harvested or having them collapse in a predetermined way when other cards are harvested and removed).
- variable player powers and/or skill trees (eg. more ways to manipulate cards like swapping or destroying, ways to manipulate the position of the other players, ways to slow down their ship or harvesting, etc...). Also you could explore the use of cosmic spores to upgrade the motherships (in a very Metroid-inspired fashion).
Again, very nice effort ;D