I have written before about how much Namibia have gained from the chance to play in South Africa's Provincial competitions, and I think that for Nepal the opportunities might be even better. The Ranji Trophy, India's main first-class competition, involves 27 teams who represent Indian states, or some organisations (like the armed services and railway workers). A 28th team would be a small price to pay for a little bit of philanthropy from the BCCI, a little bit of helping thy neighbour.
Of course, the idea of philanthropy is that you personally gain nothing out of the venture apart from a warm fuzzy feeling, but for the BCCI, this wouldn't be the case. They would gain a more competitive domestic structure, a close relationship with an Associate on the up-and-up, and perhaps a tiny amount of respect from cricket fans outside India. And, as an added bonus, the annual rigmarole of trying to make the Ranji format logical could also disappear with the new format of four groups of seven (North, East, South, West) and quarter-finals. This idea really does seem to be a rare win-win.
Nepal's cricketers would jump at the chance to test themselves more regularly.
To conclude, it seems only logical that India should seek to include Nepal in their domestic cricket, not just in the Ranji Trophy, but also in the one-day (Vijay Hazare) and Twenty20 (Syed Mushtaq Ali) competitions. It would benefit both countries, and cricket as a whole.