Currently I have a GTX 970 and an i5 4690 both at base clock speed, if I replace my 970 with a 980ti, will my system be bottlenecked?
No, Not at all, i dont think you quite understand what bottlenecking is, You will be fine with a gtx 980i, you can even go sli and still won't bottleneck.
■ ■ You mean: Will my system bottleneck my GTX980ti?
No your i5 4690 is one of the best CPU's right now... Even with 2 GTX 980ti ( as my colleague's above also said) it will run fine but if your thinf about a third card (long away it think) you should consider a i7 4790k or i7 5820K but no need right noww.
Simpel: Get yourself a GTX 980ti and you will be fine for the years... (Not that you won't with your GTX 970 i think..)
Hope it help't you..