It would be hard but not impossible to extend the current Metrolink terminus from Eccles onwards, would involve the relocation of Eccles bus station however. The street running sections in Eccles are fairly fast as its mostly segregated and straight, the real slow section is the quays however because of density of stops and the number of sharp turns rises and descents. it takes 31 minutes from Eccles to Piccadilly station, an extension to Leigh would double the line length but probably add only another 20-25 minutes because past Eccles would be all segregated on former rail allignments with few stops. That compares with 35-40 minutes with one change from Atherton. Compares less favourably with Atherton-Victoria 26 minutes where tram would take same time as to Piccadilly but require a change. However the biggest boon I think would be Leigh residents access to Salford Quays/Trafford/South Manchester (where a number do work) where it would be 30m faster by tram than by train. What could make this more viable is if the Trafford Centre/Port of Trafford Metrolink line currently envisaged was built a spur from the Trafford Centre through Barton to Eccles would only take 5-8 minutes and give very speedy Eccles access to the Trafford Centre.
I dont think a Leigh-Kenyon Junction could work, the alignments nearly 100% built on in leigh itself and then outside leigh its now roads.
Best connectivity option may be a Eccles-Tydlsley-Wigan reinstatement of Wigan-Eccles line whose alignments completley intact just out of the centre of Eccles all the way to where it met the now WCML at Platt Bridge. With a delta just after Tyldsley would allow Leigh-Wigan, Leigh-Salford and Wigan-Salford direct services (maybe Wigan/Salford directs running via Leigh evenings and weekends). Then a new connection between this line and the Atherton line possibly going on to Bolton though that former alignments 100% built over in the urban areas of Bolton and Leigh so would have to be entirely new street running sections or alternative route found. But that would be end of the century kind of timescales.