Well the unlocks would have to be sufficient enough that research and land are not the end all and be all of a powerful empire. Plus, they don`t have to be permanent. For example, lets say you managed to win 3 battles in a row totaling a 9 planet win in a 24 hour period, you could get like a 50% boost to turn regeneration for a day and 0 upkeep for all your ships for like the next 500 turns. If you make this feature only available to paid, it would provide an incentive for players to buy paids too.
As you can see, a low pr and relatively new empire could easily get this just by hitting 3 fleetless planets, but it would be much harder for a 500 planet monster to achieve the same thing unless he were good or managed 3 fed war wins getting 9 planets with 3 attcks. The powerup would enable the low land and relatively new empire to (temporarily) compete with the 500 planet empire.
This is just an example, and the specifics can be modified to whatever is deemed "balanced" so PLEASE don`t get hung up on the specifics and think "oh 50% build rate and 0 upkeep for 500 turns is "overpowered" so your idea sucks". This is simply an example to illustrate my point.