Easy there, easy...LOL!!!
I don't know if brisket would be the thing to hit next or not...read up on it as much as you can, if that's your poison...
Myself and others here feel that brisket is the king of smoked beef. Very tough cut of meat, and it takes a patient smoke tender to do it well. This is one which can test you to your limits, so don't rush into it...you'll know when you're ready to try it...it'll be knawing at you for days, begging for you to take the plunge...it is a long smoke, especially if you grab a 15+ lb packer, so plan for an early start and a late morning or mid-day getting to bed the next day. I've had more than one take well over 18 hours.
I've been a brisket fanatic since I learned good methods this past 6-8 months.
Anyway, sounds like you're ready for a more challenging smoke. Butts are a good thing to start with because they are pretty simple and easy.
Hmm...what to smoke next...there are so many things...whole chicken or spatchcock chicken...chickens are a bit harder to do well and get the way I like them.
Aaaaaah, I digressed...must be tired again...LOL!!!
Eric