Hello, HoTD fans ! I am from Russia. In my country here no many HoTD arcades, but I played second game arcade in Sochi as a kid and saw third and fourth games in Moscow five years ago. So I fell in love with series and watched walkthrough of all games (including Overkill and Zombie Revenge, but not EX). I played Windows version of HoTD 2 and HoTD 3 too.
I so sad that we don't have HoTD 5 for ten years. Overkill is great game for sure, and it expands series overall story, but without tarot-named bosses and without ties to main series besides G I can't take it like "true" HoTD game. It was not on arcade, I guess too.
So I got some fantasies and guessed about series future.
1. Bosses and design. In my opinion, because all bosses are somehow connected and use similar elements (Like Hermit and Lovers, Judgment is mix of Hangedman and Chariot, etc.) there are my designs of last three bosses.
1a) Moon is Sun counterpart, so it will be great to add another stationary boss in pool. To connect it with other games it will be great to mix it's design with one of most stand-alone bosses - Tower. Since Sun is plant, Moon can be big fungus with little snakes (like Tower's) as it's mycelium. It can hold many orange snakes, who will attack player, and some blue snakes, who must be killed to damage boss.
1b) Priestess. Since Empress is mix of Emperor and Strength, Priestess is mix of Hierophant (Priest) with someone else. If Hierophant was water-themed boss and summoned piranhas, Priestess can be so fat and impervious (like Temperance) with only sometimes opened weakspot (like Hierophant), be fought in swamp (call back to Lobber) and be producer of zombie-frogs (like Mother created zombies). Additional points to make it's weak spot look and located like vagina, but I'm not sure if HoTD 5 will be 18+ arcade...
1c) Devil. It is big question, since i have theory of him being fourth Heavenly King. Because of it, his weak spot will be on his chest; he will use pyrokinesis like Magician; his design will include more red and he will have horns, perhaps, like Wheel of Fate. If using theory that Four Heavenly Kings are ressurected big bads of series, then Emperor is Clement Darling (maybe), World is Goldman and Wheel is Curien. Because of it, Devil can be Harry after his face-heel turn and/or maybe Mystery man himself.
2) Another guesses about Magician and Star. They are so similar to Heavenly Kings, but they made from flesh. I think they are Fake Kings, who will play their greatest roles in final chapter of series.
2a) Magician. He is fan-favorite, most recurring boss and character in series. Tarot card of Magician means gaining experience and overcoming obstacles. Because Magician is one of some creatures is series, who get his own consciousness and intellect, I think he will be FINAL BOSS of the series again. Maybe in final chapter he will overthrow Mystery Man and his creations and somehow will get great power, that make him much, much healthier and harder to kill. I want to hear great remix of his theme too.
2b) Star. This character is great reference to Dante. He was pictured as not heartless guy in EX. And he is very similar to Magician. So, I think he can be brought back as accomplice of Magician in HoTD 5, but after some point he will made heel-face turn and help heroes in they fight with Magician somehow. Or he will be just boss of one-to-last chapter with some redesign.
3) Because of Goldman's phrase in one ending of HoTD 4, I think that HoTD 5 will take place somewhere in Germany, and story will be about hidden castle of Mystery Man, who will try to release Devil and/or his new creations made with help of Daniel Curien's vital, but will be stopped by heroes. Then Magician appear and he will take role of ultimate villian, similar to his killing of Curien in HoTD and Clement Darling killing Papa Caesar in Overkill. Lisa and Thomas Rogan, G and Daniel Curien will be acting, with Sophie Rogan and Isaac Washington cameos. Bosses will be: Moon, Priestess, some returned boss(es), Devil, Star or some another returned boss(es), new great form of Magician with few stages (like World).
So it is my guesses about long-awaited fifth and presumably final chapter of HoTD series. I like to hear your own guesses and theories, some corrections and disproval of my thoughts.
P.S.1 - I hope my thoughts, partly translated with Google translator were understandable for you.
P.S.2 - Just for fun, one of my fantasies is some form of crossover of HoTD with another franshice with great use of Tarot motives - Persona (Shin Megami Tensei). I think that Magician could become great guest boss in some Persona game, or HoTD RPG with SMT mechanics could be great spin-off game.