I generally stay away from them as a rule, but many of these games have a reputation of tapping into the human weakness for gambling addictions by offering random amounts of currency needed to unlock newer and better heroes at a ludicrously slow rate, and then offer more reasonable odds for a price. These tend to be time sinks for freebie players and money sinks for those willing to spend money.
Of course, much of the issue is that Gacha games pretty much evolved on the mobile environment, where these practices are pretty much the status quo. The game mechanics are built around gambling tendencies, but it could be possible to adapt it a bit to more standard monetization practices that won’t get you burned at the stake. Actually, one model would be to look at Traveler Tales’ Lego games. One feature is that you get to collect a large collection of characters (Heroes, if you will) from the franchise you can play as, many with unique abilities (besides the “wow” factor of being able to play as Darth Vader). While many could be unlocked just by going through the story mode (including heavy hitters like Indiana Jones, Superman, and, surprisingly enough, Darth Vader), most of the villains and NPC characters like General Grievous and Galadriel had to be purchased with the in-game currency of studs. However, all of these studs could realistically earned through in-game methods. Even the ludicrously expensive characters like Lex Luthor could be shortcut if you budgeted enough to invest in stud multipliers. While these games may have been for kids, there was still a lot of reward in putting in the hours to purchase your favorite villain through what feels like entirely your own efforts. The only price of admission was the $40 dropped for the CD in the first place. A perfect hero collection game, with old-fashioned, reliable monetization.
If you are ever in a need to expand your monetization options in the future, you could always try DLC packs that add new characters to unlock. Just be sure to add other stuff like additional campaigns as well, to avoid the micro-transaction and battle pass stigma.
TL;DR: Play Lego Star Wars. That should give you some ideas.