But when you make a character with a lack of an imagination whose main power is imagination...
I don't think Hal's relative lack of imagination is particularly a drawback. He can fly, he can punch, he can shoot, he can put up shields. When something special needs doing, like a building is falling over and he needs to construct some sort of stand to prop it up temporarily, he can whip up something that will do the job. He can do all the basic fundamentals, and do them well. I think Hal is actually quite well suited to being an introductory Green Lantern character.
Kyle Rayner is on the other side of the imagination coin from Hal. Kyle was a comic book artist who was given the ring, so his creations have the most creativity and flair (self-indulgent much, comic book writers?), but that doesn't necessarily mean Kyle's constructs are better.
If Hal were to make a simple gun and fire it at some sort of measurement target, the "power level" of his shot could potentially be near-infinite, but the real-world factor in determining it's power level would be "How powerful does Hal
want this gun to be, and how badly does Hal want it to be that powerful?" If Kyle were to shoot at that same target...
...he would create a Patlabor-style mech that shoots the target with it's hand lasers. But that's just extra style and flair. It doesn't change the measurement formula on the power of the shot, which remains "How powerful does Kyle
want this Patlabor hand-laser to be, and how badly does Kyle want it to be that powerful?"
Basically, Hal is a good basic starting character for introducing Green Lantern to new audiences, while Kyle is where you go after the novelty of Green Lantern starts to wear off. Also, Kyle is harder to write, because his constructs require genuine creativity on the part of the writer.
...and put him against a character immune to his imagination, it makes things weird without a talented writer
Substitute "imagination" here with "superpowers".
Hal has superpowers. You put him against a character that is immune to his superpowers, and that doesn't create a problem requiring an exceptional writer, that creates a standard conflict, requiring a
competent writer.
Also, just because Hal uses his ring in somewhat mundane and non-flashy ways, that doesn't mean his brain is deficient and he's incapable of improvising his way into victory after he's backed into a corner by an overwhelmingly superior opponent.
