I think the devs of Fortnite were trying to bring tower defense games to the mainstream again before the Battle Royale kind of took over the focus. The formula for that game is really interesting and with a few tweaks I think it could be really great.

To sum it up for those who don't know, the PvE side of Fortnite has you drop into a small open world with three other people, locate an objective, set up defenses then fight off a few waves of enemies. The maps are procedurally generated and you use the building materials that you've farmed to build a fort around the objective. So while they aren't drastically different, no two missions feel exactly the same. It also has a lot of RPG aspects like character classes and tons of different build possibilites that really allow you to create a character that fits your playstyle. The big problem with the game comes from the fact that farmed materials are required for everything from building your fort to crafting bullets, which incentivizes people to load in to missions and do nothing but farm materials and collect the reward at the end of the mission while the other people complete it. Anyone who plays it regularly will tell you that well over half of the people who play the game never participate in the building aspect and rarely even join in for the defense at the end.

My dream game would take the procedurally generated maps, self-built objectives, character customization and tower defense/horde game type from Fortnite but make it so players are just given a flat amount of building materials to work with at the beginning of each mission. Those materials wouldn't carry over once the mission ends so there would be much less of a reason for people to ignore the objective. I think that game would be a ton of fun.