Do you think Talla Tarly (Samwell's sister) would be a good candidate for Willas's wife? Her father is one of the Mace Tyrell's strongest bannermen and if something should happen to Dickon, she's next in line to be Lady Tarly. No to mention, she's half Florent and marrying into the Tyrell family would help heal the rift between the Reach houses. Not to mention, the two brainiest guys from the Reach would become in-laws.
If Mace were interested in making a match for his heir with one of his own bannermen, perhaps, but I don’t think Mace was. His own marriage to a Hightower, and his father’s and sister’s marriages to Redwynes, placed the two most powerful reacher Houses squarely in the pocket of House Targaryen. Randyll Tarly is no Godric Borrell, to be sure, but he’s also no Paxter Redwyne or Leyton Hightower; he’s not bringing so considerable wealth or power to House Tyrell that could not likely be matched by other of the “second tier” reacher Houses. Talla’s position in the succession to House Tarly is minor at best (and no less so once brother Dickon marries Eleanor Mooton and starts producing heirs of his own), and the prize of Horn Hill not a particularly grand one for the Lord of Highgarden even in the best case scenario, where she does inherit. Her Florent connections might make a neat historical union (especially since the Tarly daughters seem to be the only unmarried girls of close Florent descent (besides Shireen Baratheon), but that somewhat tenuous connection alone would not help the Tyrells assert their dominance over “those dreadful Florents” or be worth giving away Mace’s most valuable marital prize.