First, it's WAY too early to worry about rain! Second, I really wouldn't worry about rain at all. It happens. Getting wet isn't the end of the world. But a few planning tips can help:
Crocs are pretty good in rain. Teva's and the like are also a pretty good option. I also like Ecco three strap sandals (not cheap, and not super great for heavy rain, but able to tolerate a little wet, and very good support). Sometimes cheap flip flops are okay for short usage. They are also compact/light. Crocs come in many styles, but may don't offer all day foot support.
If you can, do invest in relatively new sneakers. (break them in) Old sneakers smell very bad when wet. I've never had to do it, but I read, if they do get wet- stuff full of newspapers to dry them. (WDW has USA Today) One year ours did get wet, and because it was pretty damp every day, they did not dry for several days (before I read that tip).
Also, something called moleskin can be very useful for blisters and the like, though you need a way to cut it up, because it comes in sheets at pharmacies.
Plastic ponchos get debated here. I think one-use cheapos are the way to go. Years ago, we bought WDW ones, but they have gotten rather pricey since then. We still have them somewhere, but they are bulky to pack/carry. Cheapo ones fit in a pocket.
We also have lightweight windbreakers form LLBean. Technically they aren't rain jackets, but they work in light rain. They weren't much $ and fold into one pocket. Other companies make similar ones. (We also started a tradition of buying WDW charms to go on the zipper tab.

). I dunno, I don't fret getting wet in the parks all that much. It is hot, you are wet, so what? Go swimming? The only time it bites is if you have a nice dinner ADR.
Heat is probably the bigger factor. It pays to take an afternoon break, until thunderstorm o'clock passes. That is also the downside of having ADR's in Epcot's WS with the new no-show fee. I DO suggest making plans to eat at your home resort. Escape the worst of the heat! The afternoon storm usually only lasts an hour or less, so simply taking an afternoon break is an easy way to avoid it. Then again, one August it was so cool, we wore long pants at night! Too early to fret about weather. Get up early.
Then again, we didn't find the Poly the best resort for that time of year. At Poly, going on the monorail means you HAVE to brave the elements on uncovered walkways. That's why we liked CR much better. From CR, you can get to ANY dining on the monorail loop without exposure (except Narcoosee's), and arrive to dinner looking civilized.
Then again, we swapped Sept trips for August trips. August = lots of late night hours. After the storm passes, FL is nice.
We usually like to walk at WDW- say Epcot to HS - because it is faster. In hot, it helps to take the boat/train that loops MK. If you have to make a transfer that would be complicated (Via WDW transportation options), suck it up and either get a rental car- or take a taxi. It is money well spent.
Also toss in a few opportunities to escape the heat. We don't plan it, or do it every day- but sometimes non-MK, you can get walk up lunches in TS places with AC. You might also throw in a few indoor shows....but....I WILL NOT wait 45minutes standing on blacktop to see the Nemo Show at AK. See the China movie in Epcot- that kind of show.
Wrapping stuff? Again, I don't worry too much about rain. We DO carry water, and wrap the water in 2gallon Ziploc-style bags.
Otherwise, in summer, it REALLY pays to carry the minimum. Carrying a backpack is miserable in the heat. (sweat factor). If you know you are taking an afternoon break- - just bring enough water to get through to 1pm-ish, and rain ponchos. instead of autograph books- consider just a few sheets of paper that you later glue into the book. I don't carry what I buy- I have it delivered. I don't buy much, but WDW will deliver items to your WDW resort for free. If you prefer- or are offsite- they will hold it for you at the front of the park. You just have to remember to get it. Mostly we just do hotel delivery - or - we only buy stuff late in the day - on our way out of the park.
We don't bring a lot of snacks, but if we do it is things like oranges and granola bars wrapped in waterproof wrapping. You could use the 2-gallon ziplock.
Last summer, I found a backpack cooler. It isn't very big, and did keep water cold. It was also fairly waterproof. The only problem was - like I said- it was backpack. We didn't carry it every day, or all day. (It was more useful for our spring hiking) the brand is embark and I bought it at Target for about $20. It also has has very nice padded straps. Those little drawstring bags get uncomfortable all day.
Avoid mister fans. They make us feel sticky, not refreshed.When not raining, it pays to actually try to stay on the dry side. Wicking materials.
Do bring plenty of sunscreen. (apply it as you are leaving your hotel room) And water. Drink LOTS of water! Request extra ice water with every meal, and refill your plastic bottle (at least at QS dining.) Label your plastic bottle with a Sharpie. I have a midget Sharpie.
I also carry a tiny tube of hand sanitizer.
The stuff we carry- I replenish at night if I need to. - like refilling the hand sanitizer tube. I just carry a tiny tube of sunscreen. (in case we forget to apply in the am; I have just enough to coat everyone.)
Funny though, last year- DH forgot one day. We found someone applying sunscreen, and offered them $2 for a squirt.

We made a quick friend in the process.
If you are staying Poly...well...you might just want to look at staying Club level! Read about it, and see if it is right for you. "Thunderstorm o'clock" just happens to coincide pretty well with appetizer hour.

The Poly lounge has a giant picture window facing MK. It won't cover all your food by any means, but it will cover breakfast, much of your beverages, adult beverages (beer/wine/cordials), afternoon sweets, evening appetizers, and late night desserts (8-10pm). (Pairs well with non-buffets) You still will probably want a light lunch, light dinner.