1..Reduce all weapon damage by 30-50% and that will ensure no plant/zombie can kill a foe with a single clip of primary weapon or a one shot kill with a special weapon.
2..Resort back to the 50% max energy regeneration.
3..Extra XP for garden capture/kills
4..Garden capture boast on the podiums.
That will ensure:
a) Loaners can no longer just run around simply overwhelming opponents with 'superior' weapons.
b) Snipers can't just sit at distance randomly shooting enemies for easy vanquish streaks
c) Team effort is required to both attack and defend a garden.
d) An increase in close combat fighting
e) Regeneration pot plants become useful again.
f) Sunflower/Scientist become more active again due to the lack of instant kills and therefore an increased need to use healing abilities
g) Players are better rewarded for playing the game as it's supposed to be played
5..Add an option to turn the visual effects on/off (that really can't be too difficult)
6..Increase the 'out of bounds' areas to lessen respawn camping
How I see it is that, due to the endless permutations, balancing an increased amount of characters to make them equal is pretty much impossible so the easiest way would be to simply alter the damage mechanics throughout the entire cast and then people would actually have to start using a little skill (yeah, perish the thought at actually having to be skilful) and teamwork to obtain objectives. Each character already has unique attributes away from firepower (some are faster, some have airborne abilities, some have more energy, some are smaller etc), so let players utilise these attributes instead of the over reliance of firepower. It's not rocket science.
Reducing firepower will also create a more fluid game, remove the chaos and more importantly take away frustration of stepping 1mm too far out of cover and getting wiped out by yet another unseen foe with a weapon that has too much damage attached to it.
And shock horror, people might actually start to enjoy the game.