Now that Ele TD has finally transitioned into Sc2 perhaps the map also has some potential to add more depth and variety into the game play on top of its new graphical polish. Specifically my idea is to add another unit type with a different ability. The ability is to spawn lower tier units upon death. As for implementation and examples let me go into detail. There are two primary paths that you can look at, one is that this replaces undead units and the second is adding this on top of all the other unit types.
As it stands now undead units resurrect with 33% life. Instead, would it be more interesting if upon the unit's death it creates two new units of a previous wave? As far as balance of income and the life totals, they can be altered to fit the games needs. For example, the gold received upon killing the lower tiered waves can be simply halved of what the undead units are worth now. (Or divided into thirds/fourths depending on how many you would want to spawn.) As far as their amount of health, their life totals combined could equal 33% of the original undead unit. (All numbers subject to change from more intelligent people than myself of course.)
The second path is, instead of replacing the undead wave type you could simply add a variant of this idea among the other waves. The principles stay the same in that once a monster dies, it is replaced with 2 (Or more.) monsters of a lower tier. As far as which tier is used, well that is debatable. You can choose the previous tier or a few waves back. Along with this there are still a lot of other options that can be explored. Would these lower tier wave monsters have the same abilities as their counterparts? IE if a mechanical type monster is revived, will it still have its ability to negate damage and to what end? Will it's ability remain the same or be nerfed? This is the part that of the idea that I find the most interesting. Recycling monsters whether it be limited to include only their elemental properties, abilities or the combination can really change up how someone approaches the game.
When you compare this ability to the others it becomes clear that it has the potential to be far more overpowered in difficulty however, this peaked a final idea. If it is indeed too powerful, have this ability restricted to a mode/difficulty level. If anyone finds this post interesting and plausible I will elaborate as to my interpretations of the other current wave abilities and how the mode/difficulty setting would work.
Thanks for entertaining my primitive imagination!