I think the 13 will essentially be a repeat of the 12 but with all the kinks of the 12 "ironed" out, like a much better 5G modem that doesn't murder the battery, a bigger battery, better Wifi, better Face ID, better cameras, slightly better display quality(not a new type, just better quality), etc.
Really "just" a spec bump all around but might end up being a really solid iPhone since it's not a jump in design, displays, cellular modems.
For me I'd think it was a pretty good upgrade if we just saw a larger battery and maybe more RAM in the non-Pro, standard iPhone 13, like up to 6GB from the 4GB in the current one.