Should we reconsider Data.Monoid.<> if we can't make it work with the
pretty package?
I don't think so. Monoid is much more important then pretty. I also think we need to stop trying to make globally unique identifiers. It doesn't scale. Stop it. :)
That and any other name we pick is just as likely to be fraught with similar issues.
If we really do insist on re-bikeshedding this, I think the only other viable contender is the existing infixr 5 >< from Data.Sequence. Then you could encourage people to switch but not break anyone who imports both Data.Monoid and pretty. (and it becomes easier for me to push for the inclusion of Data.Semigroup later, since the name wouldn't conflict).
Personally, I would be fine with either moving to >< or going with infixr 6 <> and just bumping the major version on pretty and sending out a warning about the behavior change, just so long as we actually do it. We've been talking about this for 2 and a half years now. I'd just as soon be done with it one way or the other.