Simple multiplayer idea:
Two players on the same map, each competing to be first to finish two out of three quests.

Each player has the same 3 quests to work on, and each has their own progress counters for them, but once one player finishes a quest, the other player can't get credit for that quest any more.

There are equal numbers of tiles reserved for each player to build on, but also additional "neutral" tiles that are first come, first served (you can still only sell your own built towers/modifiers on such tiles). Both players get coins for each kill regardless of whose tower got the killshot, but only the tower that got the kill will get the xp.

You therefore have to balance long term map control, tower upgrades, and quest choice better than your opponent does.

This can also be converted into a cooperative play mode by simply converting the quest competition into one or more cooperative single-game quests to complete together.

When a competitive or cooperative game is won: maps with harder quests yield better rewards. Harder quest maps are only accessible as your skill point total meets certain thresholds (i.e. easy quest maps are accessible with 10+ skill points, while legendary quest maps are accessible with 200+ skill points). The rate and/or quality of rewards earned in multiplayer should noticeably outpace what would be achievable during a similar time spent in solo play, to warrant the effort of relying on other players for progress, and warrant the time invested in gaining the necessary skill points.