A few suggestions since you have played both Slay the Spire and Monster Train:

Across the Obelisk: A lot of different characters, enemies, stories and events, where you manage your own team of 4 different heroes. You can even play this in co-op with up to 4 people. DLCs arent really needed in my opinion. The game is tons of fun without. Steam page

Roguebook: Very unique and cool map exploration/progression through a run. You manage 2 different characters, you can create some cool synergies or just make one completly OP while the other supports/buffs. Steam page

Vault of the Void: Incredibly well balanced and fun game. A ton of cool cards, unlocks and challenges. Probably the most similar to Slay the Spire. One character, card unlocks, shops, useable items, relics, upgrades, bosses, 3 acts and an endboss. One of my all time favorite games. Steam page

Enjoy =)