I haven't sketched out a detailed plan yet, but I'll get to work since you asked
Appearance: Make it look like a flying insectoid
Attack: Launches a stinger that deals 20 damage, or 40 to the head. Stinger can, of course, be coated with poison using a booster.
Attack delay: 1 second
[There should be an adv version worth 3 evos. This one should be able to have more hp, deal more damage and dash quickly in the air (sort of an airpounce) as well as use a trapperlike sticky -spit alt attack.]
Why you believe this could add to tremulous: The aliens lack a long-range harassment unit. This will discourage humans from simply backpedaling and force them to develop new evasion strategies.
Why you think new strategies should be forced to develop: First look at starcraft two. The first and second games were pretty boring. The third update however, received critical praise from critics and pro-gamers alike. Why? The update introduced only 6 new units. But these 6 new units caused unimaginable harm to those who weren't prepared to face them. For example, there was this alien which launched a high-damage blob of magma into the air, its location marked by a flashing red crosshair. Players with good micromanagement (that is, being able to manage each individual unit to its full potential) easily evaded the falling projectile. Less skilled players were however quickly devastated by the one projectile ripping apart their army. All in all, change is fun.
For example in trem, we always have the 45-minutes rant-fests every game. My concept could diversify the early game and change it, however it would need refining.
Since this post is disorganized, I'll make a fork post (split) soon