I personally feel that Nitish, should in the long run, do what naveen patnaik, did to BJP in Orissa. Hat's off to BJD's strategy and conviction, that has really paid of so well. Nitsh today or later will have to part way's from BJP, as the very nature of both parties ideologically is different. Nitish is seen as a secular person, with no leanings towards any fundamentalist issues, while BJP, until it does a full image makeover, will always be burdened with its legacy of mandir and masjid, which for people of India, is no more than rhetoric.
In the long run, Congress and JD U , would always be more in sync as alliance partners.
Right now, Nitish, can't take the risk of elections in Bihar, for vidhan sabha, but the election being due in two years time, Nitish should look at parting ways, sometimes before the elections.
This would lead to total pulverization of RJD as a party and give congress also a heads up in Bihar, with Nitish's impeccable legacy.
Or Nitish should go all alone, but in this era of coalition politics, to have a say at national level, alignment with one of the larger national parties is also needed.