With Dogecoin being bias to PoS, is there any alternative to increase profitability, once LTC becomes a lone reward?
The pool is already merge-mining multiple chains, not just Dogecoin. The number of chains one can merge is practically unlimited, so Dogecoin dropping AuxPoW wouldn't really open up a spot for another chain.
LTC+DGB for example?
That cannot be done, since neither coin supports being mined as an auxiliary chain.