There is a new Roslyn language server that's poised to be a replacement for Omnisharp, and it's been included in the C# extension for VS Code. From my research so far (mostly what's been written at neovim/nvim-lspconfig#2657), these are some things needed for integration:

• The server only supports connecting via named pipes. On Windows, Emacs has no native way of doing this (Windows named pipes are substantially different from their Linux counterparts). The elcord extension uses a PowerShell script to work around this by converting it into a stdio process (https://github.com/Mstrodl/elcord/blob/master/stdpipe.ps1). Such a workaround may need to be versioned in lsp-mode if other servers end up using this connection type.
• Pull diagnostic support (Support pull diagnostics (textDocument/diagnostic) #4276), since the server apparently does not support the older textDocument/publishDiagnostics method.
• Identifying which solution file (*.sln) to open when a workspace is created. When the user picks one, a solution/open notification is sent to the server, followed by the nonstandard workspace/projectInitializationComplete notification sent to the client. This is required for most useful features like Go To Definition to function.
• The server does not use urlencoded/hexified URIs internally, so the path/URI conversion functions need to be overridden.
• Decompiled sources are placed into the user's %TEMP%/MetadataAsSource folder, so those should be accounted for lest lsp-mode attempt to treat the file as originating from a separate workspace.
• Server install via downloading from NuGet (like https://github.com/jmederosalvarado/roslyn.nvim/blob/main/lua/roslyn/install.lua), since there are a lot of dependent DLLs that need to be downloaded.