Seeker on Surface to surface missile? Are you talking of terminal guidance?
How can it be sleeker? That shall substentially increase the heoght of missile. It shall be fatter may be with all composite. Avinash chander said that warheads released from missile may futher travel to 1000 KM. So effectively the range shall be 7000 KM + atleast.
Perhaps what i understand is the present Agni 5 being a direct derivative of Agni 3 is a bulky in size..Lemme show the illustration
Whereas Agni4 beinga derivative of Agni 2 is sleeker and less bulky as lots of weight ahs been actually stripped off as part of "sleek" missile. Agni 2 weighs around 18 tonnes whereas Agni 4 around 17 tonnes. Yet the range vs payload wise the increase in distance of coverage has helped Agni 4.
Agni 5 by contrast is a behemoth at 50 tonnes. Thus this missile becomes completely suitable only from land usage .. meaning either silos or via specialised trains. A comparision versus other missile shows a similar weight missile ranges warhead and weight.
I firmly believe Agni 6 may be taking over the Agni 4 design features. Perhaps using Agni 5 to change the mass and enabling it to fit into a submarine thereby completely achieveing the triad's lethality. Of course with MIRVs.
While researching i did find something. i am pasting it here
"In May 2008 Indian scientists of DRDO had announced that they have crafted and roped a path-breaking technology that increases the range of Indian missiles and satellite launch vehicles by minimum 40 per cent. The improved range is made possible by putting a special-purpose coating of chromium-based material to a rocket's blunt nose cone.
The chromium-based material acts as a reactive-ablative coating that forms a thin low density gaseous layer at the tip of the rocket as it touches the hypersonic level; this super-heated gas layer reduces drag by 47 percent, thereby allowing range enhancements at least 40 percentage point It has been confirmed that this technology will be incorporated in the upcoming Agni deployments after having undergone ranging and calibration tests."
Interestingly why not utilise this tech to other missiles also needs to be seen. I am sure this can enhance most of missile ranges by a good max 40%. The only question how many missiles of india achieves say hypersonic speed at terminal ranges or during flight? any details would be appreciated.