What's Next?
I am of the opinion that the 1983 Basic Set frequently referred to as the "Red Box" version of Basic is the single best introduction to the hobby of tabletop role-playing that has ever existed. The combination of readability, a programmed learning approach, and solo and beginner adventures, both included in the two volumes, taken all together make this a most excellent product for beginners, "aged 12 and up". The Red Box contains some useful advice even for more experienced players and referees, and I still find the red cover booklets to be an enjoyable read - almost 40 years after I first opened and experienced that new red box edition of the Basic Game.
Like previous versions of the Basic Game rules, Red Box follows a format established by J. Eric Holmes in his 1977 Basic Game by limiting the Basic system included to playing the first three levels of character experience. For players seeking higher levels of play, and more powerful characters, monsters and magic, the publishers recommend consulting either the Expert rules or the Advanced Game system. I am going to break with this advice and suggest the next place to go after mastering Basic is to the independent publisher products, specifically I suggest one give a serious look at the Index Card RPG Core, published by Runehammer Games.
Index Card RPG is a personal vison of how to make The World's Most Popular Role-Playing Game a more playable and dynamic game - written a very gifted and dynamic person! I think publisher/author Runehammer/Brandish Gilhelm succeeds admirably in achieving this vision. Mr. Gilhelm takes the familiar "basic game" mechanics off the beaten path by combining new ideas and fresh advice regarding just how to create your own setting, how to leverage the maximum fun potential in the game, and how to keep things moving along at an exciting pace while playing an awesome fantasy adventure tabletop game. The attributes found in many of the better third-party publisher products often address one or more of these topics to be honest, but none do a better job of it in my opinion than ICRPG.
The truth is that independent publishers in all fields of endeavor can take things to "eleven" because they are not overly concerned with answering to the stock-holders bottom line. They often publish unconventional works by new and upcoming artists who are experimenting and are unfettered by conventional practices. They express a level of creativity that would make the corporate types "uneasy". If you want something new and different, I have found the independent publishers are a good place to look for such.
To illustrate my point, I will give one example of creative thinking to be found in Index Card RPG - I turn to magic, which is of course one of the defining elements of any "fantasy" genre product. ICRPG uses magic items (mostly discovered through adventuring) as the basis of the game's magic system in the ICRPG game. The referee will stock the setting (dungeon, etc.) with various and original magic items, many are single use items such as scrolls and potions, but often they are not from the standard list of spell effects. Finding (or at higher levels, creating) such items empowers the player character with "magic" that is novel and interesting and ultimately at his/her/their command, at least until the magic runs out and the item becomes inert. Gone are the classic spell-by-level tables one finds in many versions of the game (including Red Box) as ICRPG does away with such mundane magics. Spell casting in ICRPG is by reading a scroll, or by pointing a wand, or by activating that precious ring you found in the goblin's lair. Of course, one can combine the two magic systems by using a blending of Red Box magic spells and ICRPG item magic to take your game to the next level!
There are many more creative innovations beyond a unique take on magic to be found in ICRPG and a referee aspiring to run creative and fun games can pick and choose among them all, adding what seems most useful while retaining as much of the original system from Red Box as seems appropriate.
Worldbuilding is an aspect of the hobby which I particularly enjoy and the advice and game world examples found reading through ICRPG are some of the best ideas I've seen. If vanilla fantasy is your goal, you probably don't need (or want) the sort of information found here, but if originality and playing in a world of open potential in setting design is desired, ICRPG and many other indie market game products are an excellent place to look.
Where one takes their game once the Basic Set rules have been mastered is a matter of personal choice. Do you seek originality or something tried-and-true? Fresh or familiar? If having a real "choice" matters to you, I highly recommend exploring one, or preferably many, of the creative offerings to be found outside the corporate publishing world.