Start with the decks that did well at the Professor Cup last year.
Donphan
Arceus
Garchomp
Then, find other decks that utilize similar thoughts as those decks. Specifically...
Donphan: Quick, cheap, powerful attacks with tanking ability.
Arceus: Sniping with acceleration.
Garchomp: Sniping with TecH support.
Donphan was powerful in the Prof Cup because it could get damage going quickly, even utilizing a starter Pokemon, and could TecH itself for certain matchups. Mine ran Glalie to grinch other Donphans and to take advantage of my BTS spamming. I also ran Jirachi as a Starter and Rapidash 4 as a Kingdra counter (which I sadly never faced). This is the first deck that most players think of in 30 card because of its speed, power, and tanking ability.
Arceus is probably the most flexible deck for the format, because it breaks the 2-for rule, has a built-in Trainer speed engine, and accelerates energy like nothing else possible in the format. It's also the toughest to get the build correct. Which Arceus are the best? How many Normals should I run to ensure the acceleration? How many LvX? You need to have a good amount of energy to ensure the triple snipe, but you also need to ensure that turn 1 accelerate. What's the correct balance?
Garchomp is the easiest to build IMO because you simply use what you know about 60-card to build 30-card. However, your possible TecH list changes considerably because of the necessity of quick and powerful attackers. I'd steer clear of the other LvX SPs and focus instead on your consistency Pokemon. ToxiTank is always fun. Ambipom is downright nasty. You'll probably find some SP Pokes that you never even thought of before for this deck.