Pat Cash needs to calm down a bit. That said,
Factors which will decide if Fed maintains his 2017 performance next year:
1. The back. If it's ok, he can still do well and be in contention for the big titles, but I don't think he'll repeat the ridiculous how-to-win-the-AO-and-Sunshine-Double-at-age-35 magic trick again, no matter what. But a Masters title or two isn't out of the question, especially on the faster surfaces.
2. How well Nole and Murray come back: if Nole is back to his best immediately in 2018 (unlikely, but possible), then all bets are off. Roger can usually handle Murray at the majors, but a fit Murray is more than capable of beating 36-year old Roger anywhere. There is Stan too, but he has never beaten Roger off clay, so that is less of a factor.
3. Whether Zverev, Kyrgios and Co. can finally bring it at the majors. Zverev will probably win one of the hardcourt majors in 2018. Shapovalov would be interesting to watch, but he won't be ready for major success for another couple of years.
4. The draw Fed gets at Wimbledon: If the draw isn't overly tough, and if his back is ok, Fed is perfectly capable of winning SW19 again. He cut through some very good players at 2017 SW19 without ever getting out of third gear. His grasscourt nous and variety is still unmatched by anyone else- in my opinion only Nole at his very best will be capable of stopping Fed if he's anywhere close to good fitness and form. But a lot of things can change from now to June 2018.
In summary- the 2 hardcourt majors and IW/Miami will probably go to Zverev, Murray, or Nole in some combination.
Rafa will run through RG and the clay season again, unless Nole/Thiem really step it up.
Fed, if he maintains fitness and form, will have a shot at Wimby and some of the indoor tournaments and Cincy.