If the main worry is about writing stuff in fanfiction similar to what the author might want to explore later himself, you should be relatively safe with the OMB universe. You might want to stay away from resolving the Keichi/Belldandy relationship, though, as this is something that I would expect the Manga to finish with.
Of course the main disadvantage of strictly using a modified OVA continuity would be that none of the newer charters and ideas from A!MG would appear. No Hild or Velsper, no Banpei-kun or Sigel, no Peorth or Rind, no Doublet system, no Angels, no parents and no Chihiro and jobs at Whirlpool motors.
Of course the main disadvantage of strictly using a modified OVA continuity would be that none of the newer charters and ideas from A!MG would appear. No Hild or Velsper, no Banpei-kun or Sigel, no Peorth or Rind, no Doublet system, no Angels, no parents and no Chihiro and jobs at Whirlpool motors.