Let's get this out of the way first, if you just want to comment that there is no way to make it work, or that platforming games can't work on touch screens don't bother posting here. The idea is for those who see a glimmer of hope to elaborate on ways it could be done well.
I would go with something along the lines of a Harmony of Despair 2.0, with much more polish and content. You could start out with one to three free characters, each of them bringing levels related to their past games, probably built with those same pre-existing resources. Basically how Harmony of Despair was, but with more specific character levels. So if your free characters are Alucard, Simon, and Shanoa, you would get mini versions of the SoTN Castle, the Cv1 castle (possibly in its full glory a la HoD), and maybe a few OoE levels, possibly including its castle. Keep all the characters unique, like in HoD but expand on their abilities where you can. Much more customization and leveling though, supweapon and spell evolutions that go way past one or two small changes, make playing with a character for a while, grinding and collecting actually worth it.
Now, where the necessary evil of micro transactions come in... It's easy. Charge a small fee for new characters, I mean if they start off with 3, charging for the others couldn't really be complained about too much. But stuff this thing with bonus characters, there's no reason they couldn't include virtually every protagonist and bonus character in the series, 8-bit, 16-bit, 32-bit... whatever. Each of them accompanied by their own slew of game specific levels and playstyles. Make more bonus levels that can be bought, maybe specific powerful weapons, character skins, spells, items, etc... Maybe make most of them possible to attain without paying, but very difficult. And then bonus modes like the tons we're getting in bloodstained could bring in even more money.
This could even have a very interesting story, again an evolution of an idea fro HoD, you are reading through a book about the legend of Castlevania, somewhat similar to the idea in Eternal Darkness, each character and their levels just represents a different chapter in the Castlevania history. Or you could make it a time traveling kind of thing including St. Germaine and/or Aeon.
And the biggest deviation from HoD, for god's sake a dedicated single player mode. Something like a mini version of each players original game, with no time limit, and a save feature.