I played every inch of DD1 that PS3 gave us, which i am fully aware was just a small portion of what PC got, and every inch of DD2 i cared to. There are things both games got right and wrong.
IMO - mana in chests and the forge will terrible ideas. In a franchise that demands constant replaying and farming - every single second of added and forced gameplay that does not help, is a giant waste of time. DD2 had chests as well, and when that was removed, life got a lot better. Same with having a deck of builders to instantly switch from, but granted we had a dozen heroes to choose from. Please stop adding things that needlessly add time to every round, every match. Just makes burnout happen that much quicker for far too many players. Auto bag collection was another great addition. Having to run from one end of the map to the other just added time, and time that didn't help you in any way.
However, both games missed one thing that i thing the franchise needs to figures out to move into the future. I loved DD1, back then. DD2 took some very necessary steps forward, but the evolution must continue forwards or the whole thing will go stale and lose everyone. Is everyone still playing SMB1 or FF1? Of course not, we all push forward. Square understands this, as no FF games reused the same battle mechanics. Every game was a fresh take, not that they all were successful mind you.
What i think DD is missing? 3 major things. 1- a reason to vary your builds. Every map, its the same towers, same patterns, same build. Even with all the DD2 heroes we had, it was cut, copy, and paste on every map. The game needs to force you to learn every def, and when to build different. Onslaught mutators and Chaos enemies tried this in DD2, but hard counters were a major mistake to me. A better way needs to be found. Maybe environment based? Something new... 2- synergy between towers and heroes. No one wants a total AFK map, as its boring, but DD2 minibosses were annoying as hell. Rollers were a pain in the ass. Dark Assassins were lame. A way to build def that woek, but still give you a reason to run around and do anything still needs balanced out in a way that feels fun, not just forced. 3- enemy AI. Units have 1 dimension track minds, and every time you play you basically get the same game with the same effects. To really take the franchise to the next step of strategy, enemies need to react to your builds in ways that change how each wave is played. See a blockade? Maybe they backtrack to another path. The more fresh each wave feels, the smarter the enemy- the more fun the strategy to stop them becomes. Tactics that work in EVERY SINGLE TD GAME shouldn't work anymore. Force us to think and evolve.
Anyways...just some ideas...