The expansions for race for the galaxy give identical phase selection cards.  They can be used alone with no other components of the expansion to add more players.

The only limiting factor becomes the size of the deck.  Without adding the expansion more people are drawing from the same amount of cards.  This will trigger more reshuffles.

This can be meta-gated by not using cards form the top of the deck to track goods.

Also the first two expansions do not add many more rules.  After a couple of plays i strongly recommend adding them.  While the takeover rules are difficult, players often choose to not use them.  

Overall base-game+ 2expantions no takeovers, seems like the most fun the game has to offer