Disclaimer - I did NOT participate in the sandbox trials. I'm only throwing my voice in to say, whatever you do, please do not let HE (or HEAT) shells penetrate destructible environment or spaced armor/schurtzen screens. That's a solution without a problem, and it only adds yet more cognitive dissonance to the game.
If you're trying to account for how broken EBRs are (which IMHO they are), then my fixes are as follows:
1) Don't make it so easy for them to right themselves when they flip...it's like they get a "pass" from the laws of physics...punish stupid derby play that is so disruptive to all other players.
2) Want to make them fast...fine, but they're not drag racers. Moreover, in the real world, tracks exist for a reason: uneven or soft ground. SOFT terrain resistance for wheelies should be brutal compared to tracked scouts. Hard/medium terrain doesn't matter. That forces EBR drivers to be far more cognizant of their surroundings, and imposes at least some situational limitations on them.
3) Offset that with vision values more in keeping with tracked peers...currently this only exists as an arbitrary balance to their absurd mobility. Fix the gimmicky speed problem and restore some of the vision.
I have the full EBR line, but I still dislike them strongly. "Dynamic gameplay" is BS code for "decreases quality of game experience for other players".
Otherwise, HE spam on HTs - especially for HP damage, is annoying, and I'm cautiously optimistic about the implementation. Gotta be careful not to severely gimp those tanks who legitimately use HE (HESH/HEP) rounds to damage heavy armor. Those HESH/HEP rounds are usually premium and should retain high penetration values for that reason (IRL, that's how they work). However, throwing DERP (non-premium HE) should cause very little HP damage to medium or heavy armor. FWIW, I do however think it would be appropriate (and entirely consistent) to allow even regular HE to implement at least some of the same stun mechanic as arty. Crew and module damage should also remain possible with HE, albeit perhaps at a reduced rate for modules (esp internal ones - engine, fuel tank, ammo rack that cause catastrophic effects).
My 2 cents from an 8-year veteran of the game.