> What OP's "hypothetical portfolio" means is basically the tracking of a whole set of selected equities (so in your site's case: the performance of everything within a watchlist, including a graph visualizing it plus all the common features like comparing that portfolio-graph to that of the Nasdaq, Dollar Index, BTC ...). [2]

> GF's UI was horrible, but they allowed to create endless amounts of those portfolios, which was really helpful for finding one's investment strategy because you could compare "ideas". Or you could gather groups of similar equities to track their performance and decide whether they are ripe for becoming "a new idea" (like "I believe that downturn of x is at some point going to halt, maybe then it's worth investing")

I just want to second this point made in the parent comment. What I've always wanted to do that I can't really do in an app out there, is compare my "what-if" scenarios (e.g. what if I had held onto 20% of my AAPL in 2015, instead of selling and buying NFLX and UA/UAA?). I always imagined that there'd be an easy visualization comparison between two scenarios, to allow me to "evaluate" certain buy/sell decisions I've made, retrospectively.