More cells, more antennas, more spectrum are the three major tools for adding capacity. To begin:
Adding antennas is usually the cheapest choice in 2018. Nearly every telco is going to four (4x4 MIMO.) Adding spectrum is very attractive when you have some unused but expensive if you have to buy it. Adding cells was the approach before LTE-A in 2014 permitted aggregation. Since then, tower building has almost stopped in the developed world. Any upgrade should include 256 QAM, more bits per Hz.