I wholeheartedly agree that the $5 gift your guild is sorely missed, as well as the ability to get multiple deals at once (just the GyG + nerfed deal is kind of lame). And I agree that the deals have fallen behind in terms of how much stamina you get for each dollar amount. The "Flash" sales aren't very good, either, especially when you factor in how GyG works.
I also think they should start giving out newer and better items with the deals. In addition to more stamina for each level, it would be nice to get items like Cyan/Orange/Purple item exchange tickets, stamina resets, shop refreshes, and chests (soul, gold, purple, orange). Another idea: a few pieces of top-tier gear (read: gear that needs the most difficult-to-farm scraps) for purple, orange, and cyan depending on the player team level.
I would like to see these deals consistently better during special events and holidays. As shops and chest buys are often discounted, why not increase diamond deal bonuses too?
It would also be cool to see "Catch Up" deals that feature items like stamina, gear exchange tickets, double drops for campaign and xp, that sort of thing.
And, people LOVE buying deals that give them soulstones for new heroes. In my opinion that is the more palatable way to make a new hero pay to win: get 30 or 50 soulstones with a decent-sized purchase (as opposed to putting stones in a gambling chest at a rate of over 20k diamonds to unlock).
EDIT: For a chuckle, check my edit history.