There are tons of different way of going about Wacky David's moveset, so I'll share my two cents on a couple of them
Sun Meter:
This one seems to be the most obvious, and as the core mechanic that the entire gameplay of Plants Vs. Zombies revolves around it makes a lot of sense to include. However, there are a lot of ways to go about implementing it. The Sun could be used for every attack that involves a plant, only specials, only plants that you place down, or only used to "upgrade" plants, such as wallnut to Tall nut, Peashooter to Repeater to Threepeater, etc. I don't have a terribly strong feeling one way or the other, but it does seem like the most obvious thing to include for Dave.
There's also the question of whether only planting plants with his specials would use sun, or also moves like aerial which use plants as well. Im leaning towards not using sun for the aerials, but Steve's down air uses iron and does nothing if you have zero, so its a possibility.
Day/Night Cycle:
The idea of Dave not getting automatic sun on night stages seems interesting, although I dont think it would work too well in execution. It would be quite unbalanced, as he would just be weaker on some stages, and some stages have cycling day night cycles, not to mention unclear stages such as the Metroid ones and Pac-Maze. Potentially keeping him the same but with the sunshroom replacing the sunflower on night stages would be cool, but we'd have to designate which stages count as night stages.
Potted Plants/Turrets:
Like the Sun Meter, this would be a great way to emulate the Tower Defense elements of Plants Vs. Zombies. It feels natural, and the peashooter would be an obvious one, but theres lots of specifics that could be discussed regarding potted plants. I think as Addy mentioned, a dedicated special and a wheel make the most sense, instead of different plants for different specials or a random menu. Plants Vs. Zombies is all about deliberate strategy, and not a random plant selection, so a random menu loses that element of strategy imo. I also think theres room for other more specific plants, such as wallnut and potato mine for the Side and Down specials, which leads me to want to keep the "turrets" to one move.
You could select between a peashooter, cabbage pult, or puff shroom on the wheel, which would all have varying stats of power, range, longevity, and arc in the case of the Cabbage Pult. From there, like in the bejeweled minigames, you could have the option of upgrading your plants for the cost of even more sun. Peashooters can branch into repeater, which goes into the gatling pea, or snow pea, which slows, or threepeater, which fires three peas at a higher position than the regular peashooter, Split Pea for both directions, or even the fire/electric peashooters from plants vs zombies 2 which would have those effects. The Puffshroom could be upgraded into the scaredy shroom for range, or the fume shroom for power and the ability to hit multiple opponents in free for alls. Finally, the Cabbage Pult could be upgraded into the Kernel Pult for a chance of stunning the enemy with butter, or the Melon Pult for more power, which could then be upgraded to the winter melon for even more power plus a slowing effect. This end tier upgrades would obviously cost a ton of sun making them rare to get, but possible if played well.
I think its best to limit it to one "turret" on screen at once for the possibility of 8 Daves playing at once, as well as the potato mine and wall nut moves being a thing. I like Addy's idea of digging up the plant with a shovel down tilt in the event that you want to reposition your plants or remove them.
Rechargeable Seed Packets:
In Plants Vs. Zombies, not only are you limited by sun, but plants have to recharge before you can place another one. This could be another option to explore limiting the number of plants on screen at once, although I think just stopping it at one per "class" of plants plus a sun meter would be sufficient.
Money Meter:
While in plants Vs. Zombies you do have to pay for Dave's Twiddydinkies, or gardening equipment, I think we have enough to manage for the need of another meter. Plus in the game you're buying the equipment from Dave, so we can assume Dave already has them. A taunt featuring a marigold giving random coins, either a silver coin, gold coin, or a rare diamond would be a nice reference though.
