There are some rumors that after about May 1st there will be significant changes to Dr. T, HSB, and Gearheart. The rumors say:
1. Dr T. will be twice a month and HSB and Gearheart will be once a month.
2. Dr T. will have 35 levels with the final levels being more difficult than current level 7.
3. HSB will have 20 levels and there will be some "rocket rain" defense, which a player can control (and upgrade in the armory).
4. Gearheart will have 7 attacks and will have two stages. First, you will need to destroy the "iron curtain" and that will unlock the attacks on the war factory itself.
5. Each event will last for 36 hours.
We will know soon whether or not all or some of this is true. Meanwhile, if you rely on Dr. T, HSB, and Gearheart as a source of resources and parts, I would suggest to save on parts and upgrade the most important things. I personally will push for max offense.