If you really have no need for any of the Heroes currently on offer, holding onto your spare Legendaries and waiting for the next event could be a reasonable choice. As mentioned here, the Soul Exchange will be stocked with a different set of Heroes every time it returns.
Will "better" Heroes come to the Soul Exchange in the future?
There is no one-size-fits-all answer for this question, as "better" in this context is quite subjective. A Hero you would personally have no use for may be the final piece of the puzzle for others — it really depends on which Heroes you already have, which Heroes would best compliment your own roster, and which kind of strategies you enjoy running.
Please also consider that individual players' impression of a Hero may not necessarily match up with the Hero's "actual" place in the game.
All that being said, subsequent Soul Exchanges may feature non-Season I Costumes, Ninjas, and other more recent Heroes. Let's wait and see what the future has in store!
Can I suggest or request Heroes for the next Soul Exchange?
To keep things fair for all players, the inventory of the Soul Exchange is put together solely based on certain live metrics. Regardless of this, please feel free to voice your ideas and thoughts about the event on our Community Forum — we always appreciate good feedback.