Term is an issue. The Flyers have a few talented forwards in the system that could be vying for top 6 spots in the next 2-3 years, if not sooner. Konecny is likely to be eligible for UFA status in four more seasons. The CBA expires in three years, so the possibility of a stoppage looms.
Part of me thinks a three year deal makes sense at, say, $13.5m for a $4.5AAV. You're not buying any UFA years in that deal, so the cap hit should be lower. However, not sure how I'd structure if from the player's side. On one hand, I can see front-loading it so he gets a bunch of $$ up front so that he's got some security in case of a stoppage in the third year (maybe $6m, $4.5m, $3m). I could also see him wanting it structured like Werenski's deal at $3.4m, $4.3m, $5.8m. If the CBA remains in place with current rules, the Flyers would have to qualify him at $6.38m.
If you want to buy UFA years, I think somewhere around $20-22m/4 years, $28-30m/5 years or $36-40m/6 years might make sense. I just don't know if they are willing to commit to him long term.