There are going to be custom game modes for multiplayer such as COOP, 1v1, 2v2, 4v4, and 10 player battle arena + some other cool variants like Endless (single or coop survive as long as possible against never ending waves). Also a built in world builder for custom maps + c# scripting built in for even more custom worlds + mod loader for even more custom stuff.
In the current version, classes don't play a significant role, but they are extremely useful in the upcoming update as they also influence pets, towers, resources, active and passive skills, etc... Classes make it easier to expand into other biomes, underground, hell, dimensions, etc.. which are coming in next update. The classes also help with all the new 50+ bosses.
As for driving the player to explore the infinite worlds. In the upcoming version, there are 12 biomes to build into with their own resources, bosses, creatures, and need different playstyles to conquer. There are also different dimensions to explore and collect new materials from (Currently 5 in next update) which have significantly harder monsters to take down but you can get for instance, the cosmic and nebula ores. There are also different planets to explore, but this is way late game.
For progression, there is a achievement tree that has connections to the next things to do with tips to help the player get started towards completing them like farming, dimension exploring, tech, towers, magic, etc.
I had others mention the tooltip hovering and I added it in the new upcoming version. I left out a lot of stuff in the details above as they are a surprise as some of the mechanics behind gameplay are fun to figure out.
Yeah, this next update is so unbelievably large and I can't wait for the community to see it.
Thanks for the feedback!