How does one get a new game added to Roll20?

I'm interested in seeing if Battletech pen & paper could be added to roll20, is there a place to request this somewhere?

If what you are looking for is a character sheet to select when creating a game then you should put a post in the the Character Sheets forum  here  and see if someone is working on one or willing to make one. The sheets are created by members of the community.  Even without a specific community created sheet you can still use Roll20 to play just about any game. The generic character journal can be manipulated to accommodate just about any system . There is a great tutorial  here showing how to create a sheet in the character journal. It's for creating a D&D sheet from scratch with macros for die rolls and such, so the system is different of course but the basics are there. 

Like Ed S. said, the base Roll20 client will allow you to play any game with the correct manipulation. The games you see on the list aren't really "set" games, merely character sheets that people have made and submitted to be showcased by Roll20 My advice if you are having trouble with finding someone who is working or willing to make a sheet for Battletech, just use the base blank Bios and use them for keeping stats and such and try and get familiar with learning different macros. Or if you grab a subscription account you can take a little time to learn how to make your own character sheet. As long as you don't need it to be extra pretty or flashy, even someone with no html or css experience can learn how to make a working sheet for Roll20 and its wiki resources. Not to mention as a DM the subscription benefits are extremely helpful for running your game.

I'm interested in this question as having a game listed in the pulldown menu makes it easier for people to find in the player/game search function. As Rod E. asked, is there any way to request a game be added? I'd like to see Shadow of the Demon Lord added to the list.

To get the system listed in the menu, it has to be more commonly posted recruiting either in the LFG forums aka in demand or be seen playing on the servers in a noticeable amount by the devs (look at the  Orr Group Industry Report: Q1 2016) . The devs are trying to keep the listing of the game systems being actively played somewhat organized (it was a disorganized mess before they started this method) and they do it by basing their selections on what is being actively played. D&D is the big boy on the block which means for any one other game being listed, it appears that 10 D&D games are listed (exaggeration but you get the point) in the listing and LFG forum. What it boils down to is get a demand for the system out in the public and it will get added. You can also create a suggestion and get members to vote on it also. For your suggestions, we offer the Suggestions & Ideas forum, which is based on voting from the whole community. Please review our Forum Voting wiki page for more information on how voting works, and our Suggestions & Ideas Posting Guidelines for the added expectations above and beyond our standard Code of Conduct . As this is more of a suggestion thread outside the designated forum, the thread will be closed.