- reduce reclaim % (scale % inversely with tech level, like 60%, 40%, 20%, 10%, for t1, t2, t3, t4 respectively)
This I think is needed. People are punished too much for using T3 units and so much of the meta around the use of T4 units is about where the reclaim ends up. Reduce the reclaim that is left and attacks become a lot more economically viable. This would also be an incentive to tech up (so you don't give so much mass to your opponent). We would see a lot more aggression I think.
I don't like the other suggestions. Watering down T3 or boosting its DPS isn't going to solve the problem.
Apart from land units, air/navy/buildings should probably keep leaving the same amount of reclaim. No reduction based on tech level.