I know this may be a little off topic on "rain", but it may go hand-in-hand with what you all are saying. Instead of introducing a "rain shield", which, IMHO, is retarded, you can introduce a "wind shield" as mentioned a few times. Instead of it being bloat and worthless and only usable in certain levels, here's how it can be beneficial in all levels:
Alright so... assuming the dev has already introduced a "Wind Shield" into the game that has leaves blowing around or whatever visual they want:
-In normal situations, it can deflect tangible, material projectiles (in other words, not lasers or energy weapons)
-In normal situations, it can deter air-enemies from getting to close... maybe make them turn around quickly and dipset.
-In a rainy situation, it can "blow away" the raindrops on screen (circular outwards of character) to improve visibility. It would also include the above "normal condition" actions I listed.
-In a rainy situation, it can improve traction on wet ground. I don't think Sonic will break too well when the grounds wet; perhaps this will make the game physics return the "coefficient of friction" back to normal? (Smidge, don't lambaste me for using the incorrect term; I don't want a lesson in fluid dynamics )
-In underwater situations, it can either be ineffective ("blows out" like the fire and lightning shield) or it can act similar to above water, but less effectively (cut down on radius of protection, blow water enemies away, knock away physical weapons, etc)
-In space/final level situations, it is worthless because there is no air. Duh.
Just some food for thought. I'm open for critique.