I have trouble dealing with anyone who has good range on their weapon. Specifically Nightmare, Sigfried, Xiba and Ivy. It seems others here have trouble with the N/S match-up but as the other two get no mention, I was wondering: Are there any specific tips when it comes to closing the distance gap so you can punish more and improve your own offensive options?
Dealing with Nightmare or Siegfried isn't too bad since these characters are significantly slower than Natsu and fairly unsafe. Using Windrolls, Distorted Breezes, 66K's, 66AB's and A+B's can help close the distance on them. WR's and DB's can roll under their horizontals to effectively get in their faces. When you're in their faces, you're in control. Throw them, 6KA them, 6B them, AAB/AA6 them, PO B them, BB them, or even 4A them for example. If you have some space, you can apply some pressure with 99, 66 or 33B.
NM & Sieg rely on good spacing to maintain offense and control. For these 2, overall, stay in close, try to TC their horizontal highs (their choice of midrange, mid horizontals is limited.), sidestep their very dangerous verticals and either pressure the hell out of them or make them throw out something that you can whiff or block punish.
Ivy's a bit of a different case because Natsu's goal is the same as NM or Seig: stay in close and pressure the shit out of her, however she can cover her ass very well with 214B and 3B. The former is an awesome move: it's quick, sidesteps to the right, tech-crouches and is a multi-hit vertical. 3B is a quick, sidestepping to the right, launcher. Similar to NM or Seig, Ivy hates when people get in her face. The key for Natsu however is to stay at a distance where the Ivy player feels "comfortable" throwing out her spacing moves (i.e. 6A, 4A, 6B~2/8, 6B+K, 6A+B*, 11A, 1A, etc.), then pounce on her after a block or whiff. If you rush her, the only moves she'll try and do is, yup... , 214B or 3B. Anticipate these moves and attack accordingly. Don't forget to block 1K on reaction, another Ivy favourite.
As for Xiba, another mid-range fighter, you guessed it, stay in close and pressure. If you stay in close, all he'll really try is 6B~K or AAK, you could punish both with an A:6 or AA, but I usually use the advantage for a mid/throw mix-up. bK or 3BKK are other moves to watch out for in close. To close the distance on Xiba, play a bit of the patience mind-game with him. Try to bait out 66BBB (usually 66BB), 4B, or 6A+B for example if possible. Smarter Xiba's will likely try 22/88AA, but after you block Xiba's midrange options, you can usually close in quickly. If you sense a 1A or 33AA, A+B him to smack his hungry ass to the ground. Oki quickly to beat out GR A+B (eg. 1K, 2B, 8A+B or 4A+B even), then stay on him. Despite his good mix-ups, Xiba doesn't have that many quick moves. You'll see 6B~ 90% of the time usually when in close.
As for spacing, I'm working on Astaroth right now. He's probably Natsu's toughest ranged opponent. If you botch anything, you're screwed.