Olympus released a suite of new objectives this year, and one of them is this 10x/0.6 NA. This is probably the longest focal length (180/10 = 18 mm) objective you can get and use with a standard two-photon system and see activity in single neurons.
0.6 NA might not be enough for tissue bulk labeled with OGB-1, but for GECIs like GCaMP6, signals from single cells are easily resolved. The supplement to the recent Lecoq et al. paper has some analysis of how relatively moderate NAs can yield single cell resolution. Some commercial rigs underfill the 16x/0.8 NAs already, and are probably using something more like 0.6-0.7 NA already.
The field of view isn’t going to be 3.5 mm wide, of course. That takes more extensive customization. But for conventional systems, this objective is probably the best bet. Albeit a 5-figure bet.