One of many key decisions the Mariners will face this off-season is what they want to do with Jose Lopez. A second baseman with limited defensive range but a potent bat, the M's have lined him up at first base more and more as the season has progressed, a move made possible by an injury to Russell Branyan and the versatility of recent additions Bill Hall and Josh Wilson. Now that the M's have called up Matt Tuiasosopo--another competent second baseman who, until the past couple months, had previously been viewed strictly through a hot corner prism--from AAA Tacoma, Lopez could basically close out the season at first if the M's so desired (although it's likely they'll give some playing time there to up-and-comer Mike Carp, seeing as how the playoffs are essentially out of reach).
The knock on moving Lopez to first has been: (a) lack of an heir apparent in the system, and (b) that while his power numbers are great for a second baseman, they're not so great if he were to move to first, a position whose occupant is expected to produce Branyan-esque power numbers. By giving Tuiasosopo reps at second in Tacoma, acquiring Hall, and promoting Wilson, the Mariners have clearly addressed the first concern. As to the latter, Lopez, who currently has a career-best 23 homers and 86 RBI (he's sure eclipse his career-best--89 RBI--in that category as well), has a puncher's chance to surpass the 25 HR/100 RBI barrier that defines elite power hitters, irrespective of position. So, in short, this year's model of Jose Lopez has more than adequate pop for what you'd want out of a first baseman.
The case for moving Lopez to first becomes even more compelling when you consider the M's ancient, mediocre designated hitter platoon of Ken Griffey Jr. and Mike Sweeney, neither of whom are especially likely to return next year (although we'd grant Griffey a 2010 farewell tour if he were so inclined, if only to stuff the club's wallet for 2011 free agency and/or a lucrative extension for Felix Hernandez). And in spite of the fact that he's enjoyed the best year of his journeyman career, we're still not sold on whether the M's should re-sign Branyan, who'll be 34 when next season starts. But assuming they do (a fairly safe assumption, based on all indications), the club might consider making him their full-time DH, rather than risk a repeat of his only playing 119 games due to injury and generally sputtering in the second half.