It is not a bug that you are unable to selectively choose these features with higher version numbers. The version selection is in there at all because there is a possibility that there exists already signed transactions which would violate these new rules. We wouldn't want these transactions to become unspendable. However moving forward there is no reason to selectively pick and choose which of these new consensus rules you want to apply your transaction.
BIP68 rules and some of the BIP62 rules are applied only if the tx version is >=2 and >=3 respectively. Therefore, it is not possible to create a tx which follows BIP62 but not BIP68. If we introduce v4 tx later, BIP62 and BIP68 will all become mandatory.
Some rules, e.g. "scriptPubKey evaluation will be required to result in a single non-zero value" in BIP62, will cause trouble when we try to introduce a new script system with softfork.
I suggest to divide the tx version field into 2 parts: the higher 4 bits and lower 28 bits.
BIP62 is active for a tx if its highest bits are 0000, and the second lowest bit is 1.
BIP68 is active for a tx if its highest bits are 0000, and the third lowest bit is 1.
So it will be easier for us to re-purpose the nSequence, or to take advantage of malleability in the future. If this is adopted, the nSequence high bit requirement in BIP68 becomes unnecessary as we could easily switch it off.
The low bits will allow 28 independent BIPs and should be ample for many years. When they are exhausted, we can switch the high bits to a different number (1-15) and redefine the meaning of low bits. By that time, some of the 28 BIPs might have become obsoleted or could be merged.
(I'm not sure if there are other draft BIPs with similar interpretation of tx version but the comments above should also apply to them)
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