Somebody of the ilk of Dusan Tadic (as a long term replacement for Pienaar), who can play anywhere in the attacking midfield three behind the striker would be top class. Tadic is valued at around 10mil by Twente but he's a quality player and would be a real 'marquee' signing.
Michy Batshuayi is immense and will be a world-class forward in a few years, but I can't see Standard selling until the summer when they can get a big price for him, especially if he forces his way into Belgium's World Cup squad which he has very chance of doing atm. An extremely good alternative (and more realistic), would be Ramos from Hertha Berlin. Agile, quick, think Adebayor but good. Briliiant in the air; not the strongest but pace makes him powerful enough to cause any defence (including Bayern's) issues. Been extremely impressed by him any time i've seen him on the Bundesliga highlights this season. Price would deb inflated with it being January, and he may wish to stay in the bundesliga and impress fro Berlin in order to get into Colombia's world cup team. They're two of the types of strikers I believe Martinez will be looking at anyway.
As for midfield, I actually see this being where Martinez uses a loan again. We need a player who Martinez knows can fit into the style of play we're spanking teams with atm. One player i'd love to see us try for would be Casemiro from Real Madrid. He's their 4th choice CM right now after Modric, Illaramendi and Alonso, but he's a class act and pretty much a young, Brazilian version of Barry with (I hate to say it) Gerrard-esque passing. Would be perfect back-up and wouldn't weaken the team at all. Probably unlikely with Khedira out for the season though i've seen somewhere that Madrid apparently want Xhaka from Mochengladbach, who plays in the same position. If they got Xhaka they may be willing to loan Casemiro out for the rest of the season, and Deulofeu has proved that Everton is one of the best places for that at the moment!
Tadic, Ramos/Batshuayi and Casemiro would be an immense window for us and would really make sure we'd be challenging for top 4 come May.