This honestly sounds like it would just exacerbate many of the problems Gen 5 has brought to defensive playstyles. Specially Defensive Jirachi will be getting Pursuit trapped by Tyranitar, Umbreon beats Bronzong and Metagross one-on-one with Foul Play, and Specs Hydreigon 2HKOs almost every steel-type in the game just by spamming Dark Pulse. Steel's resistances to so many attacking types is one of the major factors in why the current metagame has managed to retain as much balance as it has, and if it starts losing these resistances, offense will just get closer and closer to becoming the only viable playstyle. Now, I doubt that this little change to the type chart will make stall unviable all by itself, but in combination with all the other stuff that's been announced for Gen 6 so far, it could end up being a serious problem.
As for Gengar getting banned, I doubt it would happen early in the XY meta since there will be other, newly-introduced ghost types all over the place in the weeks after the X and Y release. There's also the inevitable power creep, which has kept things that were borderline-banworthy in one generation, from being unmanageable in the next. However, I wouldn't be surprised if Gengar got an eventual suspect test, maybe a year or so from October.
...seriously though -
252+ SpA Choice Specs Hydreigon Dark Pulse vs
- 252 HP / 168 SpD Ferrothorn: 159-187 (45.17 - 53.12%)
- 0 HP / 0 SpD Arceus-Steel: 178-210 (46.71 - 55.11%)
- 252 HP / 0 SpD Dialga: 208-246 (51.48 - 60.89%)
- 172 HP / 0 SpD Magnezone: 228-269 (70.37 - 83.02%)
- 248 HP / 8 SpD Scizor: 249-293 (72.59 - 85.42%)
- 252 HP / 236+ SpD Jirachi: 306-360 (75.74 - 89.1%) (!!)
- 252 HP / 4 SpD Forretress: 314-369 (88.7 - 104.23%)