Nami's engagement is a little sloppy mid to late-game these days with all of the mobility in the meta.
At this point though, she doesn't really need additional damage or healing to keep up, but everyone else is so mobile that she feels comparatively weak in the engagement category for Supports (Pyke, Thresh, Nautilus, Galio, Shen, Mao all come to mind). Despite Tidal Wave having a great big range for it, her spells these days are used more as a reaction (Tidal Wave > Bubble to keep people off your ADC for two seconds before they blow up anyway).
I would propose one of the following:
1. A small Width increase to her ultimate with Ranks 2 and 3; This would help her feel like throwing it down mid-lane is a powerful choice, and not have enemies just say "lol, Imma just side-step that"
2. An additional wave for each level coming shortly after the first (At reduced damage, of course)
3. The ability to re-activate it to produce a large splash with a knock-back or knock-up and slow
Again, she doesn't need huge buffs, but she does feel a little one-dimensional in combat, and her ult arguably feels like it has the least amount of impact in her kit. Hell, Glacial Augment and Hextech Revolver have more of an impact on team fights than her best ability does.
Thoughts and opinions?