Maybe we're all missing a key issue that's missing from all TD games at the moment: 'Smart Enemies'. Think of it: If enemies could adapt to a player's/team's play-style, detect weakly defended lanes, or strong ones and adjust their attack to counter accordingly; shifting lanes, calling (limited) reinforcements etc. This would lead to a proper challenge on every map. Maybe their 'cleverness' scales the higher up the difficulty tiers the player climbs. Similar in a way to the way computer chess opponent get 'smarter'.
I've also suggested to Chromatic, that they consider adding random weather/nature effects to outdoor maps that actually have an impact on the game. For example, randomly generated clouds, which move across the sky based on random wind patterns... As they move across the sky, the clouds cast shadows on the ground below (In random places), this could variously buff enemies/nerf defenses. One can also imagine rain, that may affect fire-based systems (On both sides), lightning strikes that hit random spots, injuring heroes and enemies alike, damaging physical structures. Standing water could accumulate, and increase chance of electrocution scenarios etc. Opposite of cloud shadows, could be dynamic sun-rays, (An actual practical use for 'god-rays') which move according to the position of the 'Sun' and the moving/evolving cloud-cover... Using the stereotype of Good=Light, Bad=Dark, the light boosts our heroes/defenses, while darkness harms them; Opposite effect on enemies of course. Lightning could spark a fire, which spreads according to the variable wind direction and as such, may harm anything it touches. Wind may also dislodge/turn around/knock over defenses, slow heroes/enemies heading into the wind, speed 'em up if heading with the wind direction, affect the course of projectile weapons on both sides, blow smaller enemies off the edge of the map etc. Such randomness introduced into the maps would bring a real sense of varied challenge and realism to play; as opposed to the simple 'arms-race' grind that is loot progression against ever-tougher enemies. Something equivalent would need to be figured out for indoor maps. Hmm? Weather ain't gonna be present, earthquakes perhaps, crystalline structures that evolve and impede defense placement/enemy pathing, random agitators that sneak into the map and disrupt your defenses, niggle your heroes, or even fight on your side and attack enemies.
Actually there's a lot of potential with lightning and fire.. Say for example, a defender wants to mitigate the risk of fire catching their defenses, maybe they have a choice of metal/wooden structures/towers... Metal would not only resist the fire, but could also introduce some interesting conductive behaviors... But electro-type enemies could use this to their advantage... using those conductive properties to short-circuit nearby towers or shock nearby heroes, or perhaps they're of a type which feeds off energy, so gains a boost to their stats. Maybe, heroes can place lightning rods, to harness the 1.21 gigawatts (To quote Back to the Future!) of a lightning strike, boosting relevant defenses, or creating kill-zones by laying out pylons at strategic points in an area around lightning-rod, leading to a chained-lightning strike, seriously damaging enemies within the proximity of the arcs. Maybe metal objects cost a bit more than wooden. Maybe fire can be gathered up by dark mages, such that they may launch fireballs at heroes/defenses, or even imbue their enemy hordes with additional fire dmg. Wooden defenses thus become vulnerable. Heroes may be able to counter such attacks... Water cannon, or drenching attacks/abilities. Water cannon would be a cool crowd-control tower too eh? As used in real-life riot control.
Personally, I've always found the strategy aspect the most satisfying. Finding an optimal placement and combo-effects etc gives a greater sense of achievement than the short-lived joy of a more powerful sword. After all, how quickly the 'just balanced' game-play disappears in current grind-based systems. I.e. You pass relatively easily through the early-game, then hit a roadblock as tougher enemies are introduced, then you struggle for a while grinding for better loot. No sooner has that loot dropped, then this immediately negates the enemies and it becomes boring as you're overpowered and the cycle repeats. Gets tedious. Hence all those that aspire to afk. If the fight was endlessly engaging/challenging, more players would actually 'play' the game as opposed to just farming a setup while they go off and do something else.
Food for thought... Appreciate such effects would require some compute power, so maybe not possible on Switch for example, but I would expect Unreal Engine 4 can assist in many areas here, so maybe, just maybe :-)