1. I find it difficult to think in terms of the Imperfect subjunctive in examples which allude to an event in the future, e.g. "Ojalá nosotros viniésemos el año que viene"... Could we also say "Ojalá nosotros vengamos el año que viene"?
2. [A comment rather than a question]: I personally prefer the "-s-" option for constructing the Imperfect Subjunctive because there is less chance of confusing it with a future construction.