I remember saying last year when Lukaku left that we had two options for replacing his goals - find a like-for-like replacement, an out-and-out striker. The problem here is that it'd be very hard to attract someone at Lukaku's level and if they don't score, then our main source of goals has dried up.
Alternatively, we pack our front line with pace, a sum-of-the-parts kind of job. Dare I say like the RS or many of the other top teams. 3 players who will chase down everything and not give the defence a moment's rest all game. Spread the work between them, share the goals, so if one is in bad form/injured, the blow is lessened.
That's the kind of approach we need. Even finishing 7th last season, we were still very one dimensional. It doesn't even really have to equate to overly attacking football - we can still organise the defence, we would just have a lot more mobility and fluidity up front.