Our enemies are within 5,000 KM distance and A-5, K-4/5 fulfils all our need. We are not yet ready for A-6/Surya with 12,000-16,000 KM range. May be we can develop, test and produce secretly but no need to disclose due to political reason.
Yes, 5000 Kms is the distance between Kochi and Beijing .. so 5000-6000 kms missile range is sufficient deterrent.
We do need the flexibility to launch the land based missile from Madhya Pradesh, Andhra, Mahrashtra or Tamil Nadu ..... and have our road mobile carriers being able to target China from anywhere (not just from the North East).
Hence A-5 and A-6 are important.
Similar range missiles launch Arihant are critical. Arihant should be able to target North eastern China both from Bay of Bengal or Arabian Sea.
(Off course, Indian Ocean based Arihants could still target South East China .... but to target NE China from Indian Ocean needs larger range).
12000-15000 kms range is lesser priority from Arihant launched missiles. Even China's ICBM are largely dummies - their CEP measurable in kms rather than meters !!
Focus of our strategic defences should submarine launched MIRV missiles with very good accuracy and lightest possible weight. (our CEP is something like 30 meters but we maintain it for 5000 - 6000 km range missiles too).
In order of priority, missile development should focus on :
1. Submarine launched, range 5000-6000 kms
2. Canisterised and able to be stored with nil maintenance.
3. Cost
4. Low weight - easier mobility, more missiles per submarine
5. Accuracy (low CEP)
6. MIRV - perhaps 6 or 12 warheads per missile
Good that India is keeping SLBMs (K-4 etc) much more secret than Land based Agni.
The SLBMs are the critical missiles of deterrence.