Steem mining right now is no longer viable for CPU miners. Someone has developed a GPU miner (assume it is not an exploit). One possible way to offer the diversity of miners is modifying the algorithm to CPU-mineable only (that is not efficient on GPU). However, it is not fair to change the rule of game (the mining algorithm) so often. So, may I suggest another option, multi-algorithm.
That is, the dev can introduce one more CPU favorable algorithm (GPU-resistant, ASIC-resistant). So we can have two or more algorithms running together. Good examples are DGC and DGB. The following is the quote from DGC ANN at bitcointalk:
Algorithm Control - If the last three blocks were discovered by the same algorithm, the fourth block will be rejected until its found by one of the other two algorithms. This prevents directed attacks on the network from any single algorithm."
DGB supports five algorithms simultaneously. Quote from their ANN post:
DigiByte uses five independent mining algorithms to process transactions over the network. Each algorithm accounts for about 20% of all blocks discovered on the network. This allows for greater decentralization as currently three of the five DigiByte algorithms are ASIC resistant and much better for GPU miners.
This may be done by opening up one more witness as CPU-mining witness. Or simply allow multi-algorithm to mine on the same witness. BTW, I personally support to have more witnesses for mining, not just one witness.
One more suggestion. If the dev really introduces a CPU algorithm, I would suggest the dev to state clearly this new algorithm is subject to frequent change, so that it only favors CPU miners, and discourages the development of GPU miner. After all, with GPGPU (like CUDA/OpenCL), someone will eventually develop GPU miner for almost any algorithms with certain degree of speedup.