So, I'd prefer a cost shuffle (BP2 to T3) over requirement switching, because the current structure is very intuitive and established.
That would make T4 rush too cheap again (and thus affect team games).
Proposal A could work with some research / build time adjustments (instant BP3, slow building T4).
Proposal B could work if early Stug G doesn't turn out too strong.
Otherwise I can only think of these changes to help the Puma / Stug E, while keeping the T4 buff:
- Making the Puma a call-in again and the Stug E either a call-in or adjusting its performance.
- Requiring T3 to be made before BP3 can be researched, so linearizing tech.