RTP's fine, but it's going to limit your options, no matter how much you tinker with it. For example, my last game was set in a giant tree. That's such a specific setting that neither RTP nor any resource packs will cover it.
Honestly, I'm starting to feel that if you come to the table with an idea, and your goal is to make the best game you can, then custom assets are a must in order to make that idea work, because every pixel on that screen and every sound emitted from those speakers matters. Limiting yourself to the RTP means that you're limiting yourself in terms of expression. When I think back to my first game, I made everything work, but I had to make the mise en scene way less charged because of lack of options, scenes had to be rearranged or dropped altogether because they'd be infeasible, etc.
That said, if your goal is to make a fun game and you don't mind not staying true to your vision, then RTP is fine. Too, if you build your idea around the RTP instead of trying to force the RTP into a preexisting idea, then you can make a game that's 1:1 with your vision and everything's peachy.