Bah, I think she won't. Just because she HAPPENS to be in the same city and has some steel typed-Pokemon which also are electric type doesn't mean that she is an automatic candidatet o replace Volkner.
For one, with her 'Amphy'... well, in the GSC games its importance in being in the lighthouse is stressed - so I don't really see them doing a backflip and pulling Amphy out of there...
Also, from what I've heard, the new Gym Leader will *possibly* be a new character, from what I've heard in the uncomfirmed rumours of Platinum. Jasmince ain't new... although that depends if they will use a new character for the role, ala in Emerald with Wallace being replaced.
However, it's still possible that she could become the Gym Leader there, but find it unlikely - after all, she's only there to visit the lands. Her to move into a new region like that, and abandon her duties in Olivive City just kinda irks me too much to make it seem feasiable.
IMO, Jasmine is only there as one of the numourous references to GSC (if you look they are everywhere - Prof Elm mentioned, man who talks abotu Whirl Islands/Mt moon with Celefairys dancing, an ingame event in GSC, etc - and coding which allows one to get Pokemon from 'JOHTO' suggests a possibility of all of these being a hint towards a GSC remake... maybe). That's all she's there for, IMO.
One will have to wait and see, though.
We'll most likely go to johto in platinum and jasmine could take us there.
Right... and Nintendo would do that, instead of making a GS remake, which would get them more money, right? I doubt this for many reasons - one, Johto is not close to Sinnoh when one looks at how the regions are positioned - Kanto is closer. I doubt one is going to see three regions in the one game, and with there's a Johto more than likely there woukld be a Kanto (Pokemon League there). Memory problems, and the simple advantage of another game meaning more $$$ if made and successful - and a GS remake most probably would.
That, and I don't believe that they wouldn't have already mentioned this if this was going to happen as a selling point for Platinum. Instead, they talk of another Battle Frionter sort of thing. And the odd 'new area' - if they mentioned that, surely they would have mentioned Johto as well, it being a bigger selling feature than another route or cave or two.
Again, I think Jasmine is just there for the sake of being there, the programmers thought they'd make a BIG reference to GSC there, on top of all the others. I'm of the opinion is is also part of the many hints towards a (POSSIBLE, mind you) GS remake - whether that will happen remains to be seen.