Theres been similar suggestions to adding in new units that you gain based on points, but if it was implemented there should really only be 3 things changed at all to each new unit; weapon speed, range, and attack damage.
1. The normal marine
2. War Pig - increased damage, decreased range
3. Marauder - increased damage, decreased weapon speed
4. Ghost - increased range, decreased damage
5. Spectre - increased range, decreased weapon speed, decreased damage (range increased more than ghost)
6. Reaper - Increased weapon speed, decreased range
1. The normal marine
2. War Pig - increased damage, decreased range
3. Marauder - increased damage, decreased weapon speed
4. Ghost - increased range, decreased damage
5. Spectre - increased range, decreased weapon speed, decreased damage (range increased more than ghost)
6. Reaper - Increased weapon speed, decreased range