When will marisa get a weapon refinement?

She only came out a relatively recently, so I doubt you'd be hearing anything anytime soon. If anything, Lloyd and Navarre will be more likely to receive a refine before her since they're more outdated as f2p units.

I'll be surprised, if she ever gets one...

The Wo Dao can already be refined

Unless you're asking when she'll get a legendary weapon, in which case, who knows?

Hopefully. But according to Fire Emblem wikia Wo Dao is analogous to Shamshir, which will most likely be her prf weapon if ever she gets one.