There will be about 50 or so heart pieces to find. Bosses now re-spawn as part of the new boss rush, so they no longer drop full heart containers. In the original game, being limited to 14 hearts, and having 11 of those basically given to you, always felt like such a let down to me. I wanted to explore more with more things to discover, so now you'll have about 5x as many hearts to find (with 5 pieces making a complete heart), and even more seashells, as well. I've tested the amount of hearts and seashells over the years and finally settled on a quantity that feels reasonable and not overwhelming.
For seashells it started out at 100, but over time, that felt like way too many, and I even lost interest in finding such a large number. It might've worked in a larger world, but in a world this size it greatly reduced their appeal after a while, especially on repeated play throughs. This subject is quite hard to explain, but as someone who has played this way a lot, you'll just have to trust me, ha ha. Before starting this hack, I was like "oh yeah! 100 seashells! woo hoo!", but it turns out to not be so great.
There will be more opportunities to spend more time with the game, as you said. I'm trying to design the world in such a way that areas will serve more than one purpose. There will be backtracking, but it will all be optional and never required. So if you are willing to explore and see what there is to offer, I'm sure you'll find plenty things to keep you occupied.
One last thing, Exodus you mentioned "soon" but I just want to clear things up a bit. While I did say 90% complete as sort of a progress indicator, the scale of this project is quite huge so even that last 10% will take time. I'm always working on this when I have free time, so it will be finished eventually, but it's hard to say when that'll be.