If you want to correct yourself, you often do this using the expression 's/typo/correct'. This plugin allows you to complete the first part (the typo) by pressing Tab. The words from the actual buffer are used to complet this part. If the word can be perfectly matched the next word in alphabetical order is shown.
The second part (the correction) can also be completed. Just press Tab after the slash and the best correction for the typo is fetched from aspell. If you press Tab again, it shows the next suggestion. The language used for suggestions can be set with the option
plugins.var.python.correction_completion.lang
The aspell language pack must be installed for this language.
Add the template %(correction_completion) to the default completion template. The best way to set the template is to use the iset-plugin, because you can see there the current value before changing it. Of course you can also use the standard /set-command e.g.
/set weechat.completion.default_template "%(nicks)|%(irc_channels)|%(correction_completion)"