I think there's gonna be a big learning curve. Agni 5 is quite compact, but if you try to put more propellants to make a longer range missile, it'll quickly become heavy and difficult to manage. I think the ideal would be a missile a bit larger than Agni 5 with a 10 K range, that way it'll be very mobile and capable of fast deployment (Like the Agni 5 which was shot off a rail launcher in it's very first try). This would mean different propellants. I don't know if we are using hypergolic fuels already, but somehow feels that we may use that intensely for the new missile. In which case safety will be an extremely important aspect (if we're not already using them I mean).
@anant_s do you know anything about the fuel mix we're using? Someone told me that in PSLV we're using toxic aerosols that will kill you if you're anywhere near the thing around launch time. But not sure if we're using stuff like hypergolics etc.