This idea comes up fairly often (and it is a good idea) but the devs shoot it down every time.

From an interview a while back:

Q: Can we have PvE skill combos like how PvP chains the skill combos? The number of skills we have to put definitely has been lesser since Stormblood but it feels like more can be done.
Y: If that happens, skill combo would be too simple and easy. Additionally, PvP uses simplified skill combos because we want to pinpoint the players’ focus on broader perspective of battle than using the skills in order. PvE combat environment is a lot different from PvP, where players need room in between combos to respond to various situations in the raids. That’s one of the reasons why we are keeping the the way of skill sets structured.

In other perspective, the development team already saw that the number of skills are reaching the capacity in terms of system limit. Instead of increasing the number of skills, we are planning to replace the skills or alter the skill that are rarely used instead. You won’t expect the number of skills surging back in the near future.