There are numerous approaches out there to increase package capacity (you may want to read the review by Chamberlain et al. doi: 10.1089/hgtb.2015.140.), increase target specificity and reduce immunogenicity. for the latter you could try an immunosuppression strategy.
If I had that double hurdle to overcome, I'd just pick a different vector system, e.g. Lentiviral system, for your research question.
A good choice would be to use dene deleted helper dependent "High Capacity Adenviral Vectors" (HCadV) which offer a packaging capacity of up to 35 kb.the do not express any Adenoviral genes and are therfore considered less immunogeneic than early generation adenoviral vectors. But they are not as easy to produce as AAV!
You can find several research articles from our research group about these vectors.
please feel free to contact me for advice if you consider to use HCAdV