Although I liked the idea of modrons, I never used them in my games. Other than the modron march, just didn't see them being interesting enough to design an adventure around them.
I'd kind of like to see their appearance being reworked (more clockwork, but Geiger-inspired as being an amalgam of flesh and machinery. Still basically geometric in overall shape, but both more beautiful and frightening at the same time. The modron things adventurers run across should primarily be the "warriors" of the race).
I'd also like to see their philosophy be revised from being passive to be aggressive; somewhat like the borg - "Only true unity with the universe can be achieved by the removal of self. Your culture will be dismantled, your inefficient laws restructured, your petty desires wiped away. You will become part of US. Resistance is futile." (With the likes of the ninoton and such being like Seven of Nine..., Locutus being something like a decaton and Primus being like the Borg Queen. If they want, they can blame the change due to the events with Thanatos/Orcus).
As for the slaad, if they go back to them preying on anything they encounter - including each other if there's no one else around, that's all I'm concerned about.