In short, there will be lots of opportunities.
First of all, for S/4 HANA on-premise, you'll still have ABAP stack and you'll still write your own Z reports, BADI, etc. One big new addition is the CDS which would be created in ABAP for Eclipse but is consumed within ABAP. This is somewhat like an extended ABAP development environment that almost all old-time ABAPers find "progressive."
Now when you talk about S/4 HANA in the cloud, it would be a little bit different -- since you won't "own" your instance, you won't be allowed to create your Z reports per se. So the level of customization would be very different from before. You will, however, be able to extend your SAP HANA with things like HCP (HANA Cloud Platform) which will give you the ability to extend your solution without "modifying" your S/4 HANA cloud subscription.
Don't forget, HCP can be used to extend your on-premise solutions too, so either way, it is a good idea to start picking up HCP.
If you are interested in expertise yourself in SAP HANA modules, then check SAP HANA course and certification.