I think one reason what they dont wanna add Wing Form is because Sieg does not fight much in the series.only twice in Final Countdown and another movie with Gaoh as villian.so they don't wanna add Wing Form in it.
I understand that, but Sieg himself is a pretty important part of Super Climax, especially the finisher. I think they should handle Den-O like this:
Green Gauge - Stay the same
Red Gauge - Choice between Super Climax and Liner
Orange Gauge - Change to Grey and make it Wing Form
This way we could all be happy. DenLiner summon sucks anyway.