Your ideas are interesting, but I have no plans on revamping vanilla Terraria's core mechanics with this mod. Plus it's already set in stone that Energy Tanks are only a crafting material used to create the first suit/starting items, and nothing more. We are, however, toying with the implementation of Reserve Tanks from Super Metroid, but nothing about them is quite finalized yet.I just saw some post about energy tanks as a "additional health point".
What about just disable player's regen, and make health being replenished ONLY by health drops (these hearts, replenishing 20 HP per one) and some kind of charger, but with CD (Chozo Statue from Zero Mission and/or Normal Charger from Fusion), and delete ability to use Health Porions?
It would dring some hardcore (like Revengeance Mode from Calamity), but will be more closer to original Metroid, which I like more.
And more about Energy Tanks, they can be just used as a alternative Heart Crystals, without combining them. I saw one mod with Heart fish, acting LITERALLY like a Heart Crystal, so what about that not-so-tricky thing?