For a while I used to main Guile in Hyper Fighting and Super Turbo. I later switched to Ryu after having a laugh with Ken.
Guile bores me in IV, though I haven’t played a lot of the game yet. I’m not sure why, but I’ll invest more time with him and see if he’s still a badass. I still think he’s still a nicely rounded character however.
What could Guile use? I’ve always figured four things Guile would perhaps do well in having:
1.) An overhead. We got this in HD Remix (and its awesome) and he supposedly has one in IV though I’m yet to tinker with its properties. An overhead REALLY adds to Guile’s game in HD Remix though. It comes in real handy when chasing booms or if you’re having a poke without charge.
2.) A command throw. I always thought this would be cool. Nothing too damaging, but something that takes a small amount and hurls the character into the air setting them up for a juggle follow-up by way of a reduced damage flash kick provided the player was very nimble at charging during the throw animation. This would be a nice way of giving Guile a more reckless style if he was to figure ‘fuck it’ and just jump in and go for it without charge.
Problem being, that would give him an advantage and round him out a little too much perhaps. I dunno. A command throw would be cool tho 
3.) A dashing two hit punch charge-move. Short to medium range, hits twice close. Similar to Ken’s command kicks that hit twice up close. Something to give Guile a bit of style and variation in his combos rather than just using booms/flashes. And hey, he has no double hit moves and they always sound meaty
Perhaps make it weak and do minimal dizzy damage so it could perhaps be followed up with a crouch strong/boom?
4.) A sonic boom-esque charge uppercut. No sonic boom flies out, but a down>up charge move with punch that sees him do a two hit crouching uppercut that works somewhat as an anti-air, but makes for a nice combo up close AND leaves him on the ground.
None of the previous two ideas really alter his properties, they just give him some much needed variation and some combo finesse.
I remember when Hyper Fighting came out and Guile had his two-hit roundhouse flash kick. It sucked on small characters as it seldom collected the second hit for the knockdown, but it was awesome doing it on large characters. It just made the guy more fun to use. It spiced him up. Since super and ST Guile’s kinda grown a little stale and dull. Great character, but not a lot of new stuff has been applied to him I think it would be nice if he received a new and unique move.