I 100% feel you. Last time I went in early September I went as Finn from Adventure Time because I asked myself, "What allows me to wear shorts and a t-shirt?". Basically just had to buy a blue t-shirt, the hat, and I did buy a little stuffed Jake as well (and weirdly got accosted by some teenager at some point during the night asking me WHERE'S JAKE?! until I awkwardly held it up - super weird).
Do what's comfortable! Disneybounding is also an option, or get creative. This year my partner wanted to go as the HM tightrope walker girl, so I said I wanted to be the crocodile. I considered one of the pyjama full body crocodile costumes for about 5 seconds, then realized that was a terrible, terrible idea. Bought an alligator headband thing instead and am putting on lightweight green clothes. Cute, but I won't die in the heat.