imo, its not optimal to try and isolate +theta without a view on price &/or vol, since these can easily reverse and more, any +theta gains.
The usage of Delta Neutral strategies, by its very nature, would tend to imply that one is not particularly keen to have a view on price, otherwise they would go for the directional trades. Volatility on the hand, because of its mean reverting tendencies, can, imo, be forecast (for one of a better word) more reliably then prices could be. So times like now when IV is fairly low (for the indices), spreads could be put on which are +vega with a slight -delta bias. That way when the big moves in the underlying take place, the +vega can be counted upon to provide some assistance to these +theta strategies. Workable strategy ??