According to Dave Meltzer, the current plan for Reigns is to have him challenge Brock Lesnar for the Universal Championship rather than wait until next year's WrestleMania. Meltzer's report claims that the decision to change the plans came about a couple of weeks ago.
With Reigns vs. Lesnar taking place at SummerSlam, it would appear that the WrestleMania main event has been changed as well, although WWE could do a third Reigns vs. Lesnar match at the show. Meltzer speculated that given John Cena declaring himself a "free agent," WWE could go with Reigns vs. Cena as the main event of WrestleMania.
However, Meltzer's report also mentioned that plans could change yet again as everyone from executives to the wrestlers are said to be highly impressed with Samoa Joe's performance in the program with Lesnar and how well it's gotten over with fans.
As far as the idea of Lesnar vs. Reigns happening for SummerSlam instead of WrestleMania, I've got no issue with it. Personally, I'd rather they got to it sooner as, from my perspective as a fan, I'd rather them put the title on Reigns rather than have Lesnar tear through the roster only for Reigns to "save the WWE Universe" at WrestleMania.
As far as Joe's performance getting over, it shouldn't really come as a surprise to anyone. I mean, booking an opponent for Lesnar who wasn't around during the Attitude Era as though he's a threat to Lesnar instead of portraying him like some scrub is something fans are responding to? Wow, what a concept.