Go for basis.
It is the hardest part of SAP, but you get to see everything and often made responsible for anything which no one else is responsible for (depending on alternate competence and inclination to feel responsible for topics).
Then learn ABAP in addition (I assume you are already OK with scripting and SQL etc).
Then the sky is your limit as long as you do not get bogged down with stupid work.
Cheers,
Julius