Gatling trucks to come back. either do them as a legendary unit and make the health go way up and damage maybe some range . or do a base defender unit . make it with huge health and damage good. range can be ok but it can spawn other Gatling trucks with weaker stats of course . the more its upgraded the more it can spawn with a limit of course . give it components like faster rate of fire , a aura that can disengage other units abilities like the dark storm , or can help the other units around it . definitely range boost !!
other type of thing would just be adding 5 levels to the unique base defenders so it can bring them back .